serious question

Why is it that most guys want anal? I know some people are into it, and I have given in and tried with one ex once. I have Colitis and it ended up tearing my stomach up. Now new guy wants to try it really badly. I am by no means a confident women nor do I usually talk about this kind of stuff with anyone, but he wanted to make a sex tape and I agreed to that but I am nervous and scared about anal...not sure what to do need advice because the only outcomes I see are me getting hurt, me shitting myself right there, or having to tell him to stop and being stuck in the bathroom for hours like last time...and first guy was smaller. Please no judging or haters, need serious advice...thank you