This is really a great post. Therapy only works because it takes the kink out of simple little things like this. The mere fact you can bring this up to her is a way of healing through therapy. Just the act of bringing up something that bothered you regardless of how small or big to a person, who is the one who did it, is the act of healing through therapy. You will feel so much better and not just momentarily but by like taking a leap in your healing. Because you acknowledge a feeling right and the intensity of it right and took adult like measure to deal with it. Rather than the usual way of trauma, not acknowledging or acknowledging but putting it to the body or the mind or dissociating. You already took the first step of acknowledgement. Now is sharing and hope the end result is opposite of your fears.
On the other hand, this also shows a bit of the therapist's human/subconscious side slipped through. Even the cashier down the road knows being gay is not a lifestyle like golfing and if you want to be generous (also another form of healing spectrum), you may teach her a valuable lesson. In such, she may learn ooops, she did it again or ooops she should not do that again to another client. You pay it forward!
If you also want, to be let us say more safe, you can bring it as a story not related to her and let her pick up the subtlety of the situation by just mentioning that you do not believe your life is lifestyle but you know people confuse this or say people may think that. This way you are not 100% directing her but putting the situation in the room - the therapeutic space and a great therapist will pick on your mature way of handling and will correct her ways in the future.
These are just some options. There could be many other ways of looking but the most important thing is by the simple act of bringing this is part of the healing you are seeking.