Question

Is it a myth that guys become more attractive when they have a girlfriend?

Answer 1

Guys don’t become more attractive when they get a girlfriend, they are only perceived as more attractive. It’s due to two effects: 

1. Scarcity Effect is the cognitive bias that makes people place a higher value on an object that is scarce and a lower value on one that is available in abundance.
2. Evolutionary Psychology: our ancestors lived in small societies and life time engagement of a man to a woman meant that they had less chance of reproducing. Although it has far broader impacts on men, especially when it comes to rejection. 

Answer 2

It’s because men who are in a relationship are more confident in their behavior around women and are generally happier. So they may look more attractive. The extra grooming and fashion senses at the start of relationships doesn't hurt this view either. Also, women are more comfortable around men who are already in a relationship, so some men may mistakenly think of it as flirting.