Romantic interactions aren't available for certain Sims
So this is a weird new bug I'm experiencing.
My Vampire has a romantic relationship with an Alien, they're boyfriend and girlfriend at this point. Their romance bar is even higher than their friendship bar. But all of a sudden, romantic interactions are no longer available between these two sims in particular. But they are for every other Sim. The Alien is a Young Adult and the Vampire is an Adult, so ages shouldn't be an issue, but it's as if there is a problem with the age difference because the interactions just aren't there.
I just started having this problem as well, and this is really recent for me, so I am testing mods - my well-married couple doesn't have romantic interactions except for romantic skygaze.... I seem to also have an issue where the game thinks my sim is a skeleton (doesn't have the buff and I didn't cheat, so I am working on this one as well)
It could be a few things other than relationship scoring that is effecting your romance availability. There are various scoring methods used to determine if your sims can romantically interact with one another.
It is possible a mod is conflicting in this scoring.. it could be a bug.. it could be some other hidden problem.. hard to say. I don't own vampires so the only advice I can give you is this,
Temporarily rename or back up and move your mods folder and see if your romantic interactions return. After testing this.. if you can use romantic interactions between the 2 without mods, then you should probably start hunting down which mod is blocking it by adding them back into your game slowly.
When an engineer says that something can't be done, it's a code phrase that means it's not fun to do.