An unattractive, emotionally stunted straight male who whines to female friends and dates, "Girls only like assholes. They don't like nice guys like me." Unlike genuinely kind and intelligent men, who easily find themselves in serious relationships, a "nice guy" wards women off with his abrasive personality, self-pity, and/or slovenly appearance, and blames his sexual rejection on the failings of womankind, while simultaneously imagining himself as nothing but God's gift to them. "If only those dumb bitches knew that I'd treat them right!"
Once in a relationship, a "nice guy" tends to either be verbally abusive or extremely clingy and overbearing.
Go into any mixed group of friends. Say that the statement "girls don't like nice guys" is false. The first guy to cry, "But it's true!" is always the most unattractive person in the group, without fail.