A lot of things actually,
During my engineering, one junior had huge crush on one of my friends, they both belonged to the same city and they knew each other quite well and they used to hang around a bit. This girl was crazy about him but she never happened to reveal her feelings for him.
Days passed and my friend started to grow closer to one of the other girl in my class, the junior girl felt jealous and one fine day she revealed her love for him. My friend very politely declined her proposal and promised to be friends with her till their stay in the college.
A couple of weeks later, everyone at the hostel and college started hindering and teasing my friend by calling him gay. Whenever he used to be in college premises, girls used to murmur in each other's ears and giggle looking at him. He couldn't really understand what was happening around. He was very sad and shattered. Many of our mutual friends started to maintain space with him, he sort of slipped into depression.
The egoistic girl who had crush on him later went onto confess him about the rumor she spread about him being gay. According to my friend, she wasn't even sorry, she thought it as her victory and pure vengeance.
The poor guy tried very hard but people were very abrupt and bad-mannered to him, intentionally/unintentionally everyone liked to believe the rumor about him. He didn't complete his engineering and went back to his hometown in his second year of engineering itself, of course he gave everyone a different reason for his early departure but somewhere I felt the doltish rumor was responsible for it.
Honestly it's not about the gender, woman doing worst to man or man doing worst to woman, it is erratic in both the cases. Never take someone else for granted, you can't make someone love you back, learn to accept the heartbreaks and downthrows. You might feel little miserable by doing it but there's no harm in it, it will only make you stronger and better person. You can't be happy by deceiving someone, never give yourself a reason to hate yourself, if this happens, your life is worthless. Spread love, not rumors.