The more I think about it, the less problematic this seems to implement.
When you get to the "sex scene" stage of a romance, have a dialogue menu for the Inquisitor (just like any conversation). Your options are, to paraphrase:
"I am so in love with you, let's do this." -> Make-out, fade-to-black, sex implied, romance continues normally
"You sexy. Me sexy. Let's get nasty." -> Make-out, fade-to-black, sex implied, romance continues normally
"Let's not jump into anything physical right now." -> No sex, but romance continues normally
It's just a very minor number of additional dialogue lines, and no extra cinematic work. Perhaps for certain characters (like Isabela), I could see the "no sex" option potentially closing off the romance.
It gives asexual players an option, and it gives the illusion of choice (love making OR dirty sex) to everyone else. Everyone wins.
But otherwise, it's a pretty cheap, simple implementation.