How about if the alien has a humanoid form and a non-humanoid form? Doesn't your sexual attraction end up leaking over into their non-humanoid form even if you fell for them in their humanoid form to begin with? Well.. mine would.
Alexestrasza in humanoid form
Alexstrasza in her Dragon form
I dunno.. the dragon form is actually hotter but that could be my metal bikini aversion. Still you don't need actual clothes if you are a dragon in humanoid form, it's not like you're going to get cold.. so you might as well dress hot to impress. But really if you are attracted to the powerful and beautiful Alexstrasza how much MORE amazing is she as a powerful dragon?