Are you trying to describe this in a clinical sense, or are you trying to get the reader caught up in the feeling?
Forget aliens for a moment, suppose you are just trying to say that a human woman is pretty. You could write, "Sally was very pretty", or "Any man who saw Sally was immediately impressed by her physical appearance." Or, "Sally's physical appearance tended to elicit an automatic physical response from most men who saw her." Etc. Okay, that conveys the idea, but frankly, it's not very interesting. Unless you WANT to sound very clinical and scientific, I wouldn't say it that way. I don't claim to be a great poet, but I could easily do better. Not original, but I heard once, a character says to his girlfriend, "Once I looked up at the sky and it was the most perfect shade of blue, the most beautiful color that I could imagine. Later I said to myself that it must have been a dream, there couldn't really be such a color. And then today I looked into your eyes ..."
BTW I think you'd need to justify your premise for the reader to find it convincing. I can look at a mountain or a wild animal and say, "Wow, that's really beautiful", but I don't think of that in a romantic or sexual sense. If someone asked me, "Which is more beautiful, the Milky Way at night or your girlfriend?", I'd say that such a question is impossible to answer. They're two different things. Both beautiful, but each in its own way. There's no way to compare the two. Unless your aliens look just like people, I doubt it would occur to anyone to compare their beauty to the beauty of people.