Depends on the girl. Some girls might get offended and jump to the conclusion that you are just thinking about sex and not appreciating the "moment" or whatever it is you guys are up to (like if you guys were going for a walk and you lean in to kiss and she feels a hardon, she may think that you are trying to do more and may not like it). Some girls don't care. And some girls will even feel flattered knowing that you are sexually attracted to them.
Rod Steele