@Castiel no recently added mods :-\ @Alistair_32 definitely not a relationship problem because the game allows all other romantic interactions! and it allowed them to become boyfriend/girlfriend.
UPDATE: I think the issue lies within the MALE sim rather than my female sim. I'm thinking this because I just had her successfully woohoo with someone else (someone she barely knew, of course, just because I was determined to find out what this woohoo problem was). Sooo maybe the guy is just a prude?!...