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 [i]isn't interesting[/i] 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.