There's a difference.
While animals ( including humans ) will indulge in same-sex, sex, they will also procreate with the opposite sex producing young & continuing their species..

Homosexuals & lesbians, tend not to. They do however adopt & raise unwanted children.

Why do some animals ( & some people ) get involved in same sex sex?

I'm told they ejoy it.

And they should be perfectly free to do so, in my humble opinion.