Though it's seldom intentional on the part of the "cheater", all girls who have large breasts due to excess adipose tissue throughout their bodies are (rather chauvansitically) said to be "cheating".
Kevin: No, you're so blatantly cheating.
2)Mark was always so busy and Amy wasn't getting the attention she wanted from him so she started hanging out with Dave. He was giving her the attention she had wanted from Mark. She may not have realized it but she was emotionally cheating.