The problem with being bisexual is the confusion. Not so much within yourself, initially there is just as much confusion, denial and recrimination as tends to go with sexual realisations of all kinds, he knows with certainty that he spent just as much time agonising over the issue as any of his gay friends. Rather it's the confusion that it causes in other people. People, as Dumbledore would no doubt remind him if he were still around, are terribly found of categories, they like you to be one thing or the other. Wizard or Muggle. Human or Monster. Straight or Gay. Things that are somewhere in between make them uneasy. If one thing has remained constant in Remus' whole existence is that he's always lurked in the in betweens.
He's unsurprised by the way people define his sexuality by that of those he loves. If he is involved with a gay man then he's gay, if he's involved with a straight woman then he's straight. Trying to explain that he's somehow both of those things and neither of those things is often more trouble than it's worth.
Romantic entanglements were never something he'd expected to have to worry much about. His primary concern had always been to fit in as much as possible, to keep people from finding out what he really was for as long a possible. Finding friends who accepted his lupine alterego was unexpected and wonderful, but if it raised any hopes of finding someone to love who would love him in spite of what he was, he squashed them firmly and immediately. However, it turned out that the heart, much like the wolf he shared his body with, was altogether harder to suppress entirely.
He'd never intended to fall in love with one of his best friends. Let alone to act on those feelings. He'd run the usual gamut of repression, denial and guilt, rationalising his feelings as hero worship and brotherly affection, an excess of teenage hormones focusing on the only avenue open to them. An overly physically affectionate, utterly accepting friend. It had worked, to an extent. Right up to the moment said best friend had stormed into the room in a towering rage, announced his own homosexuality and the challenge that anyone with a problem with that should 'have a go now and get it over with'. The silence afterwards had been silent and stunned, though when he looked back on it not exactly surprised. As though his small audience had been expecting this announcement just not the manner in which it had been made. Remus had broken the silence in an unexpectedly dramatic manner by grabbing Sirius firmly by the shoulders, shoving him hard against the nearest wall, announcing 'Thank Merlin for that' and kissing him thoroughly. Of course things would undoubtedly have been less complicated all round if the room in question had been their dormitory rather than the Gryffindor common room, but there was no accounting for Sirius' over developed sense of the dramatic.
In spite of his very public outing, Remus had never really considered himself as being gay in a more general sense. He could look objectively at both men and women, and consider their relative physical attractions decidedly pleasing. He didn't as far as he could tell have a preference along gender lines. Actually as far as such things went he seemed to have a preference towards Sirius and that was it. Sirius had been thoroughly fascinated by this notion and experimented with it thoroughly, exploring the merits of both genders with a diligence that both amused and infuriated Remus. He always returned to Remus though, he might enjoy men and women equally, but he enjoyed Remus most of all.
There are twelve years of darkness that he does not wish to remember. Of blood and spit and broken promises. Taking brief comfort in the arms of those who either didn't know or cared far too much about what he truly was. Gender matters little in this dark place, only that they will not turn him away.
Being in the Order again is complicated. When Sirius is alive he slips easily into his old role. Nothing is spoken, not even implied. The remains of the original Order presume and accept him as being the way they remember him. The newer members potter around in blissful ignorance and admire his devotion to his damaged friend. Afterwards, he can feel mild disapproval regarding Tonks from both parties, on one side the old guard with their unspoken implication that he should put her straight about the fact that he wasn't. The newer recruits unable to comprehend how someone who wears their loneliness like a winter coat can flee from such freely given affection. They do not understand (though she does, and that somehow makes it all so much worse) that having loved and lost so dramatically twice over he cannot face doing so again. He fears, not that he would not survive another loss like that, but rather that he would.
She finds her way into his heart regardless. Steadfast and true, wearing her broken heart on her sleeve like a badge of honour. She gets inside his head and his heart, a new best friend, a new love. Not a substitute or replacement, something new and different. He fears she will be tarnished and defined by what he is, a beautiful young woman dragged down by an old monster. Instead, oddly he finds himself beginning to be defined by what she is. It's both liberating and terrifying all at once.
He wonders sometimes if he should find it fitting, ironic or disturbing that the two relationships that would define his identity to the rest of the world should both be with members of the same twisted family tree. When she's feeling particularly frivolous Tonks like to tease him that about his aristocratic taste in partners, other times, she'll just shrug and claim that a fondness for Remus was the beginning and end of she and Sirius' similarities.
He worries, sometimes, watching her sleep. Watching her hair colour flow and change with the currents of her dreams, he suspects that all his fears are well-founded. That he is too old, too used up by life for her. That the devotion she lavishes upon him with such ease and readiness, is something with which he cannot compete. He fears he cannot love her enough, as much as she deserves. In the small hours he suspects that if he truly loved, he would let her go, not allow her to bind herself to him and his broken shell and outcast life. He'd fought this thing between them for so long scared of hurting her with his sharp edges, he'd eventually broken in the face of her depression unable to watch someone else he cared so much for fade into darkness and drink. He still tries sometimes to find ways to let her go, but he cannot manage it, he has grown as dependant upon her love as he once was on Sirius'. She calls him a coward and battles his demons in her attempts to help make him into the man he wishes he could be. She understands him, too much it seems at times, much as he would claim understanding is all he's really ever wanted in the flesh it terrifies him. He suspects that underneath it all it is that refusal to let him face them alone that makes him love her, in spite himself.