Cas has explained it to him; that now that he is with Dean, his remaining angelic traits compel him to take care of his mate, to breed him up and start a family, even though there is no physical way for Dean to get pregnant. Castiel’s instincts still demand him to try, though, so his sexual drive is through the roof and he can’t do anything about it. Well, except for depositing loads and loads of come inside Dean.
(Cas produces much more come than ever; it’s insane.)
And as soon as Dean understands what is going on and consents to it, there is no holding back on Cas’ side anymore; he simply goes ahead and tries to goddamn breed Dean. For a couple of days already, although his success is rather limited, no matter how often they do it. Which is pretty often. And it’s pretty much the same every time: Castiel seeks out Dean, if they are not already together, and loses himself to his single-minded purpose of knocking him up.
He grabs Dean when he tries to get up from the couch to take a shower and presses him back down. His cheeks are already ruddy and his eyes wild, yet he still tries to asks Dean is he can go ahead, and Dean easily enough gives his consent by pushing his ass against his swollen crotch. It takes only seconds for Cas to pull down Dean’s pants, to free his cock from his own, to put his hands low on Dean’s belly and deeply sink inside in one go.
(Because Dean is, thanks to the constant fucking and the excess of come always wet and open, and the knowledge that this is only because of Castiel seems to make him even wilder. Dean guesses it has something to do with angelic mating, maybe with how they are equipped for it and how Dean’s body might be like if they were actual angels mating, but he doesn’t care to ask. Or more like, he always forgets to the moment Castiel snarls and preens at Dean easily accepting him inside his hole, still dripping and loose, and when Cas then tilts Dean’s hips up with a purpose and just fucks any logical thinking and the ability of speech out of him.)
The hand on Dean’s belly is just another thing. Cas has never done that before, outside of his weird mating time, but now he’s obsessed with feeling all over Dean’s belly, gently and reverently. With both hands, unless one of his hands is straying again, higher, so that he can tenderly tug on Dean’s nipples, stroke the warm skin and cup his chest like there are actual tits there. He often pulls Dean even closer when he’s exploring his non-breasts for a longer amount of time, all up against his hard and rutting body, just so that he can lift one of Dean’s arms and bend down beneath it to kiss the sides of Dean’s chest and to suck at his nipples. It’s weird, in so many ways, but it also feels strangely good, and after those times, when he treats Dean more like a woman or whatever is going on in his mind, he is especially gentle with him. He often brings Dean to orgasm again then afterwards, sucking and kissing on his chest, two of his fingers languidly sliding in and out of the mess that he made of Dean’s hole, just fingering him with a feral smile and his face buried in Dean’s chest.
So yeah, Dean doesn’t mind the hand-on-belly thing, even if he isn’t particularly fond of the tummy he has, because the fact that there is just the slightest bit of pudge probably turns on Cas even more when he’s like this. And anyway, at the end of the day, when Dean must feel like nothing but a girl fucked loose and pumped full, Cas’ hands often leave his tummy and go to Dean’s hips in the moments before Castiel is going to come, and he pulls Dean back against himself, hard and unyielding and spearing him open with how deep he gets like this, and fucks in, roughly and with a deep growl, before he floods Dean’s inside with come. And when he’s done, he still keeps Dean like this, still and obedient, and fucks in in small jabs, even if it’s not pleasurable for either of them anymore and Dean might be moaning with it and whining about it being too much, but it looks like Cas wants to make sure that Dean got all of his semen, that he is successful in his attempts of breeding him. Sometimes, Castiel stays inside so long that he gets hard all over, and when he does, he never fails to fuck Dean again, to fill him up even more.
(Just yesterday, after a particularly thorough session in which Cas had fucking bred him three times, Dean complained about all the come pumped inside and how it’s such a bitch to get it out of his body and clothes afterwards. Castiel laid him out on his belly, kissed his way down and promised to swallow every single drop of semen leaking out of him. And he did; he got Dean to relax, and for more than an hour, he licked away every trace of come that made its way out of Dean’s hole, even going so far as to encourage him to help with the process by pushing, which Dean objected to. But Cas still licked and sucked relentlessly, with his broad and warm tongue, also kissing Dean’s balls and slowly jerking him, though he didn’t allow Dean to come. Only after Dean had proven by pushing outwards, to his absolute shame, that there wasn’t any of Cas’ seed left inside him -- and when Dean was already in tears from the prolonged yet denied pleasure and the embarrassment -- did Cas bring him to his release; with one hand on his cock and his tongue up Dean’s ass. Needless to say, it was one of the best orgasms of Dean’s life.)
When Castiel’s mating time almost comes to an end, Dean confesses with a blush that he is probably going to miss it; he doesn’t explain in further detail what exactly he’ll miss, but it’s the feeling of being constantly wanted, of being sated and fucked real good, of having his body so open and ready for Cas, of being worshipped and taken such good care of constantly, of always having this bodily connection with the guy he wants to be for the rest of his life. Sure, he is raw and sore and every inch of his body has been marked, but he also feels wanted and accomplished and loved.
Castiel seems to catch the meaning of his words well enough because he promises to show how much he desires Dean even more, and that with his mate nearby and possibly never pregnant, Dean doesn’t exactly have to worry about a lack of mating seasons happening in the future.