Because this dude at least likes other dudes enough to let you blow him, you cannot automatically assume that his relationship is the stereotypical monogamous kind.
Those rules are slowly starting to become antiquated, though, as more and more people are experimenting with different forms of relationships-and queer communities especially have pushed those boundaries. The automatic presumption is going to be that the only sort of relationship anyone can ever have or be in is a monogamous one dictated by societal norms and people are reacting under that assumption. You don't know and definitely no one here on Reddit knows. They might have an open relationship they don't discuss with anyone as he experiments. Your friend might have told his GF that he was curious about his sexuality and she's okay with him being with other guys but not girls. Here's the thing: the only people that know the rules of your friend's relationship with his girlfriend are your friend and his girlfriend.
