Hey there, and welcome to TeenHelp!

Unfortunately only you can decide if this is a red flag or not. I would say, objectively speaking, that it's a bit concerning when a partner exhibits transphobia (which, based on what you relayed here, is what happened). Unfortunately, a lot of people on the LGBTQ spectrum are not okay with others who are part of the same acronym. Some people still feel weird about trans folks.

Ultimately it's up to you how much "slack" you give and what you're willing to tolerate. If you are very against homophobia and transphobia it sounds like his stance doesn't fit your values and that may be something you need to think about when it comes to the longevity of your relationship. I'm not saying to break up with him just because he's transphobic but maybe keep the issue on your radar. People do change. He might experience positive growth in that area. But if he can't show acceptance and it's a must for you you may want to reevaluate the relationship.

Good luck and PM me if you need anything else.