As many others have said, orientation is about who you love
not what activities you engage in with your significant other.
Some analogies you can use with your narrow minded friends.
Let your friends know that a lot of lesbian women engage in strap on play, with one women receiving vaginal penetration from the other. That doesn't mean they are secretly hetero
. I know some gay men who enjoy masturbation tubes that look like vaginas, that doesn't mean they are secretly hetero, either.
Many women, lesbian and straight, enjoy vaginal penetration, this act is not exclusive to hetero women. Many men, straight and gay enjoy.... sticking their dicks into things. That isn't confined to only straight guys. Also many men, and women enjoy being penetrated anally. (Lots of nerve endings, as people have said. More for men because they have a prostate, more than even women. But we can access our G spots with anal play.) In fact, some gay men don't even use anal stimulation in their sex. The specifics acts don't determine sexual attraction.
It's WHO you love and want to have sex with, not what activities you like to engage in.
I'm a hetero female in a hetero relationship. I
like to be anally penetrated by a man. Does that make me..... a gay man? Nope. I love men and like to have sex with men. It's who you love, not what you do.