This is a really interesting question.
Denying someone else's sexuality is linked to them having increased internalised homophonic, which causes all sorts of issues. The LGBT community are at increased risk of MH issues, self injury, suicide etc.
It can lead to someone denying their sexuality to themselves, meaning they live a lie. They can end up in a straight long term relationship but having gay sex behind their partners back.
Living a lie makes us vulnerable to any crime where blackmail helps the perp. Romance frauds, sexual abuse, domestic abuse etc are things the community is at particular risk for.
I can't emphasise enough how hard it can be to have this inner turmoil, shutting down "gay thoughts", sex with people we're not interested in etc. Then there's the shame, the shame of our sexuality and how horrible we can feel. Fortunately most of us IME accept it nowadays