By it's very premise, which requires the woman in a match to send the first message, Bumble assumes that its users are straight. And as a lesbian woman who's spent a fair amount of time on both Tinder and OkCupid, I can tell you that the apps aren't great at weeding out men who don't belong (sorry for the immediate swipe left, Scott, Todd, and John, but I don't know how you got here).
Of course, that doesn't mean that queer women have to stick to meeting a new love interest in lesbian bars (which are dying, anyway). In fact, I met my current girlfriend on a dating app, so I can 100% confirm that it's possible. All it takes is knowing the best ones to download and how to work with them.