Talk about consequences if they do choose to break the rules. Make sure that your reservations come from your teen, and not from discomfort around their sexual orientation.
If you’re in doubt, talk to someone you trust to be fair and insightful about this situation.
They will discover more about how to function in romantic relationships and what they want in a partner.
The feelings, desires, and heartbreak they may encounter are exactly the same that a heterosexual teen would encounter.
Discuss your rules: no overnights with a romantic partner, no being in the room with the door closed, etc. If they don’t have your trust, how can they regain it?