The more Viggo thinks about it, the more he realizes there is nothing he can offer that Sean doesn't already have.
Talent? Sean's got that in spades. Fame? Sean's got that too, just enough to open doors and not enough to become a problem. Women? That's a joke; the last thing Sean needs or wants is another woman in his life. Children? He has those too and he's not one of those men whose sense of his own masculinity hinges on his ability to father sons.
In the end, there is only one thing Viggo can offer that Sean wants, and as Sean lays him down and opens him up and takes everything Viggo offered and the one thing he didn't, Viggo wonders if he's giving up too much for the sake of a soul.
And then, Sean murmurs his name and bends his head to kiss Viggo and Viggo kisses him back and realizes that Sean can have even this for the asking, no price tag attached.
Oh well, Viggo thinks, pulling Sean in closer. It's not like he really needed another soul.