Being friends with the opposite sex when you're in a relationship/ married

So this is a question I think you have to answer for yourself and with your partner both before and after you get into a relationship. It spans from basically being a non issue to being the reason people split up.

What are your boundaries or lack thereof with the opposite sex currently and how/would they change if you got into a relationship?

Once you got into a relationship would you expect your partner to change?

For example, if you're a woman and you start dating a guy who has a lot of female friends whom he hangs out with on a regular basis? Would you be comfortable with that? Should you be expected to be comfortable with that? Say, they tell you they're uncomfortable with that, how much should you be expected to pull back?

Let's get into this