I think you have the concept a little confused. When rating something, it doesn't have to involve the sex or gender of a person. You can rate other things as well. Moreover, when rating something, we usually do it on a scale from 1-10 with 10 being the highest (good). and 1 being the lowest (bad). Here is an example: Me: I just saw the new Star Trek movie last week. You: Was it any good? Me: Yeah, I liked it. You: What would you rate it? Me: About an 8. So, in your example above, if you LIKE the way the other person looks, you'd give them a ten. Here is another example: Me: The new guy in accounting is very cute! Her: Really? Me: Yep. He's easily a 9, maybe even a 10

You are correct to emphasize the word "one" in this sentence, just notice that the correct form is with the indefinite article: "I'd giver her a one." (meaning "a score / rating of one out of ten.) On the other hand, without the indefinite article, and pronouncing "one" without emphasis... yeah, it sounds like he's going to give her... something... unspecified.