2 gays and a straight

I'm a straight white male 34 and for well over 2 years share a house with 2 lesbians. I grew up with Irene and have been friends with her most of my life, never finding out she was gay until she was about 20 years old. Her partner is Clara who is a few years younger than us. My girlfriend knows I live with them but has no inkling of how comfortable we are around each other. It took many months to get to the comfort level we now have and in a way it is intimate. I never have sex with them for obvious reasons but the lack of modesty around each other is nothing but amazing. The house we share is two bedrooms but only one bath. I can't remember when we stopped locking the door and none of us is embarrassed to use the toilet while another is in the shower. It was never a problem if we saw each other undressed and the longer we lived together the more often we saw each other naked. It's gotten to the point now that I see the two of them naked almost daily and they see me naked as well, often. We all smoke weed together and many times I have not only watched them make out but have sex on the living room sofa. For more than a year I have masturbated while watching them and they are well aware of it.

They are amused when I get an erection and there are times they purposely get me aroused by their actions. Its become like a joke to them as to how quickly I get a hard on if, for example, they do a dance in front of me while naked. They certainly know I'm straight and find it amusing how their nudity gets me aroused easily. They question why since I know I'll never have sex with either of them. They also question why I am naked in front of them so often since nothing I have is of interest to them. I am convinced they just don't understand how a straight guys mind works. I'm so at ease around them that I often masturbate in front of them. I have tried to explain how jerking off with them watching me is a turn on for me but they don't get it. Its strange I know the way we are around each other but I can't think of anywhere else I would rather live. I'm in my boxers sometimes but some nights I just roam the house naked as they do. Another thing is how they show their affection for each other right in front of me. I have witnessed them giving each other oral sex more times than I can count. They have no inhibitions about me watching them and yet don't understand how it gets me aroused. Both are pretty girls with nice bodies so seeing them naked is enough to cause an erection. They look at me at times as I start getting a hard on and smile shaking their heads. Irene still asks me why, then telling me she is gay and I know it. I just tell her I like everything she likes, but she only smiles or laughs. I think they really do know they excite me but don't want to admit it. I also think they intentionally try to get me aroused thinking its funny that I do.