I’ve heard this many times from heterosexual females in particular. Especially when it is regarding guys who they find attractive. Even if I tell them the guy is interested in me, or that I’ve had sexual encounters with him – some girls won’t believe it till they see it.
There are multiple gay men who have kids. For some, maybe their gender preference in dating has changed. Others may have been in the closet and not told their girlfriend/wife/mistress at the time.
But there are also openly gay men who are biological parents – so the stigma that a man with kids is exempt from being gay is false. Gay men can be parents too.