Well...you're right that your question as asked only makes sense for straight FTMs. But what you're really asking is "Does the perceived sexuality of the person I'm with have an effect on how others perceive my gender?" And I think the answer is a definite yes. I pass almost perfectly when I'm out with a flamboyantly gay friend, and almost not at all when I'm out with my very straight and heteronormative husband. I get mixed readings when I go out with both of them, depending on which one of the two the person thinks I'm 'with'.
So if you are being perceived as a lesbian and you're being affectionate with your boyfriend, odds are others will perceive him as lesbian too. But masculine clothes alone don't get you perceived as lesbian. There's a particular set of social cues that do that, and not having ever been part of lesbian culture I can't say specifically what they are but they go beyond just the gender labels on your wardrobe.
