They could very well make it so sex is an additional or optional choice. Hell, what's more likely, is that they may make it so you can politely decline sex and still be in a romance with some characters.
What's tricky is that playersexual usually means that you can romance a character after flirting/gaining approval with them no matter what your character is. If your character is ace then by that logic, romanceable characters have to be written as compatible with being with them.
But as user Nyloth said, the two "evil" characters, Lae'zel and Astarion, are written as wanting sex, and downright explicitly written as promiscuous and interested in no strings attached sex - borderline "aromantic" for lack of a better word. This may be more of an issue with Lae'zel since I feel like Astarion is charismatic enough to be romantic without actually being romantic. In Lae'zel's case, her "romance" would be something more like an intense relationship with an ally who also is good in bed, and I am very interested in seeing it play out that way. Her romance is not typical, probably not only about sex, but the fact that she wants it is important to the romance.
I think there are many ways Larian can make it so you can skip sex scenes while it being implied. They may be able to write in that many of the characters are ok with being denied sex and never physically consummating, but I feel like Lae'zel and Astarion's romance would lose something if you denied them. And I would be disappointed if they had to rewrite that aspect.
