How can you get a sexually transmitted infection as a lesbian?

Same as anyone else : Exchange of vaginal secretions, anal sex, and oral sex are all transmission vehicles for STDs whether you are hetero, homo, or bisexual.

Skin contact: Skin on skin contact as well as shared toys can expose women who partner with women to viruses and some bacteria. Hpv, HSV and trichomonas are more common infections. Some lesbian women still partner with men, so unprotected vaginal intercourse can be a source of exposure. Barrier methods such as condoms and dental dams can reduce the risk of STI exposures.

Bodily Fluids: Sexually transmitted diseases are passed to partners via bodily fluids, regardless of the sex of the partner. Blood, vaginal secretions, and semen can all transmit stds. Transmission can occur m to m, m to f, f to m, and f to f. Lesbians can still protect themselves from STDs using barrier methods, just as heterosexual couples do.