Why do some men have a tough time taking lesbians seriously?

This is from experience, and I am not meaning to generalize, but I have found this to be the case a lot of times.
Some men who talk to my partner or I (a lesbian couple), seem to think we aren’t serious in our relationship. Or we are putting it on for their benefit.
It has been suggested to us frequently that we are “missing out” on male anatomy and it had even been asked more than once if our sexuality is a result of sexual abuse. That was very insulting and suggestive that there had to be a bad reason why we’re lesbians.
The result is that we are still loving and affectionate in public, but we deliberately tone it down.
I just wondered if anyone has thoughts as to why that happens?