Do you believe homosexuality is a choice?

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I have two gay sisters. One of them first married a man, had a child, later divorced and has now been with a woman for the past 5 years. My other sister ended her 3 year relationship with her girlfriend and just got married to a super cool guy. It really doesn't matter to me who they choose to love. I'd just really like to hear different viewpoints on the subject - Is homosexuality a choice? Thanks in advance.