Surprise! The alcohol (especially the shots, maybe?) interferes with heat suppressants / birth control. The result = they needed to get home ASAP and don't even make it in the flat before they are fucking in the stairwell / on the stairs.

Either way is fine, but I tend to prefer Omega!Sherlock.

Angst, crack, smut (obviously), Mrs. Hudson is amused or not amused, Mary cares or doesn't care (but please don't be too mean to Mary, I don't like Mary hate) - whatever is fine. Mpreg or not - doesn't matter.