I would have no problem with them dating. In the strictest sense I would not consider it a homosexual relationship because it’s not. And I wouldn’t consider your friend to be gay/homosexual. I wouldn’t have the biggest problem in the world calling himself that but the problem is when bisexual people like that project their experiences onto homosexual people and speak on behalf of homosexual people and act like based on their own experience that actual gay/homosexual people can be attracted to the opposite sex. And it does end up being a slippery slope because it creates that impression when people simply say things like “I’m a cis gay man dating a trans man.” The implication is that homosexual people can be attracted to the opposite sex and that therefore there is something wrong and nefarious about all of the gays and lesbians saying they can’t be. It gives credence to calling us bigots when we’re the ones who are just homosexual and it also gives credence to homosexuality being an outdated word and concept which is why some people are suggesting it’s offensive.