Honestly the best way to turn down someone is just to flat out say no, because no, because I don’t want to, because I said so. I am not interested in you, no. You’re by no means obligated to include your reasons and if they insist tell them “because you’re an annoying pest that don’t know when to quit.” That way nobody can really come up with shenanigans to force you into relationship/sex with them. No one can say well I identify as a woman so you a lesbian have to fuck me or else you’re transphobic.