Some time after BvS, Clark finds out a part of Kryptonian biology he didn't know about yet ... he goes into heat. (Feel free to handwave why that didn't happen earlier to him, maybe Kryptonians only mature sexually at a later age or whatever.) Maybe Bruce finds out because Clark has disappeared to isolate himself, maybe Clark actually comes to Bruce because he knows Bruce is resourceful and can maybe help him. Would prefer if they weren't together yet so there can be all the angst about whether the other one really wants it when they eventually fuck (for hours and days until the heat abates).

No mpreg please. Just Clark who desperately needs to fuck someone, hard and long, again and again. Clark who's out of his mind with lust, who feels terrible for ~making Bruce do this~, who feels even more guilty because he's wanted to sleep with Bruce for so long. And Bruce who's wanted Clark and feels bad for what feels like taking advantage of his biological impulses.