You can tell him, insist, refuse to take no for an answer, AND be sexy at the same time. Ninety percent of guys will be so turned on you won't be able to peel their tongues off your clit. The 10 percent of guys that won't be turned on, well, they aren't guys you wanna be sleeping with anyway. He has made it clear he wants a relationship with her, even though she's told him she is seeing me. This chafes me, and I feel disrespected.
Straight men seem to have no respect for lesbian relationships.
She goes over to his house every once in a while to party with him and his friends, and never comes home until the next day. I think that I have a right to be a little worried that something is going on. If your girlfriend is fucking this guy, you have valid and obvious reasons to be concerned. But even if she's not fucking this guy, you still have valid reasons to be concerned. If you should be concerned about anything, you should be concerned about how little your girlfriend seems to care about your feelings.