Straight men looking to write smut with someone else don't do it merely for the artistic quality of the prose they're writing. It's a fulfillment of a sexual fantasy. If they're straight and their writing partner is a man that might break the spell of sexuality of the whole thing, because to them it's only exciting if it's written with a female writing partner. They fantasize about the idea that what they're writing is also turning the female writing partner on. Because they're straight, it simply isn't interesting to them to write that kind of content with a roleplayer they know to be a guy. It defeats the entire purpose.
That isn't sexism. It's heterosexuality.