I'd say that what you have been doing until now, when your roommate is not there is the respectful thing to do.
You could have a conversation about how to make it easier for both of you to have more time alone, for whatever it is you want to do. Be it masturbation or writing a diary or reading a book in privacy for once.
As per masturbation advice, that's not really what we do, but I'm sure there's plenty of stuff online. So long as what you do is safe (e.g. cleaning toys and using condoms when needed) and respectful to anyone else you are sharing space with, I'd support that.
Also enjoying anal stimulaiton doesn't say anything about someone's orientation, it is just a part of our bodies reacting to stimulus.
"In between two tall mountains there's a place they call lonesome.
Don't see why they call it lonesome.
I'm never lonesome when I go there." Connie Converse - Talkin' Like You