I mean, you could similarly reason that bisexuals aren't welcome (both gays and lesbians are solely attracted to the same sex, after all), or that asexuals aren't welcome (you can be asexual and heteroromantic, after all), and so on. I think, ultimately, that unity between us is important, and allowing the umbrella to protect all members of gender, romantic, and sexual minorities strengthens the overall cause rather than weakening it.