They assume that ugly women will try harder in bed, be more loyal and be more appreciative. The problem is, most guys who are like this tend to be jealous and insecure. They also tend to cheat I don't care how bad looking you think you are, no woman deserves to be with an insecure, jealous cheater!
However, this is not to say that an ugly girl can't find a nice guy who is attracted to her personality. It happens all the time! There are many happy couples in this world where one person is seen as "attractive" by society and their partner is "ugly" and this is probably because these people have great chemistry.
Chemistry has nothing to do with looks. Sometimes we are sexually attracted to people who are not "hot" in a conventional sense. The good news too is that men are MUCH less critical than women think! Her hair color needs a touch up, her shoes are ugly and her butt is fat! The truth is, most men are h0rny and it doesn't take much to turn them on.
Most single young guys would do anything just to have a nice girlfriend who gives them regular sex, even if she isn't the hottest woman on earth. There is always some desperate guy who will get with an ugly girl An ugly guy on the other hand has much LESS of a chance of finding someone. Even most ugly girls won't go for an ugly guy. They would rather be single! There IS the occasional guy who "only" goes for good looking women but men like this are rare and they are usually jerks anyway.
Rather than worrying if you are attractive or not, why not just look for a guy who is into you. If a guy likes you and flirts with you then obviously he does find you "hot" or else he would be talking to someone else.
Not all men are looking for "hot" women. In fact a lot of them would probably be happy just to have an average looking girl talk to them. I hope this answer helps!