Why am I attracted to my friend's girlfriend if he is always complaining about her? How do I stop thinking about this?

My friend has been in a relationship for three, almost four years. He always complains that his girlfriend is jealous and possessive, that she is worried about losing him when he is not in the mood for sex and that she is too sensitive. I imagine it's stressful for him, but I wish I had a girl like her. My exes, particularly the most recent one, never showed jealousy and were not very passional during sex. I don't want to act upon it, but now I imagine myself in a relationship with her and how I would have sex with her, as he suggested that she is has a high sex drive and is very eager. I wish that since I'm similar to my friend in terms of looks and personality (that's what my ex told me), his girlfriend could be attracted to me too. He's been my friend since the beginning of high school and I would never do something about this, but I when I imagine my dream girlfriend, she pops into my mind. I refuse to go out with both of them now because I can't even make eye contact with her because I feel dirty and inappropriate.

Why am I attracted to my friend's girlfriend if he is always complaining about her? How do I stop thinking about this?
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