Non-gay people can go into the bars too. How appropriate that would be depends on the bar. Some are more like neighborhood bars, others are more meat markets. Being a man and going into a lesbian bar can be awkward, depending on context (it's usually OK if you're a guest of a lesbian friend as long as it's not a separatist bar, not so OK if you're coming in on your own and aren't known to the clients or bar staff, and very much not OK if you're hitting on the clientele). Context matters.