As for forced marriages ...
You've got to remember that at the time, a girl who didn't marry who her family wanted couldn't go out and get a job, and if she's going against her family's wishes, she couldn't go stay with relatives or apply for assistance.
You don't have to hold a knife to someone's throat to force them to do something. The threat of being homeless, with no protection whatsoever, and no support -- well, many women had the choice of doing what their families wanted or being thrust into a much worse situation ... if you could call it a choice.
If the family viewed the daughter as an asset to better the family, rather than a person whom they wanted to be happy ... the girl couldn't even count on societal support. That would go to the girl's guardian, along with sympathy for having raised an ungrateful, disobedient child.
In the middle ages, being in the clergy was often a MUCH better future for anyone not of the aristocracy than they could otherwise aspire to. They had shelter, food, a place to belong, protection, medical care, and the chance to learn, and a measure of respect. It was a step up, and there were fewer 'spots' open than there were those vying to get in (or vying by proxy to get a family member in).