Thank you everyone, I am starting to get a better understanding now.
Spending less time alone or with friends. Having to do things for someone else. Having to spend time with someone else's family (maybe). Having to share your money. Having less control over some aspects of your own life. Having less personal space. Compromising on little and large things. Investing a lot of emotional energy into someone else. Not being able to have sex with other people (and often having to modify other related behaviour like flirting). Generally less autonomy.
Thank you for these examples, I think it is these sorts of things that have had me confused. Several of them are things that I would never expect someone I was in a relationship to do, such as:
having to spend time with someone else's family - wouldn't expect this. My immediate family can be a little odd and I wouldn't insist on a partner or boyfriend spending time with them unless they really wanted to
Sharing money - I am self sufficient, live alone, can afford to pay my mortgage and bills, and to socialise and to go on holiday. I dodn't need a guy's money. As above, if he wanted to share, then fine, but if not I don't need his money and don't expect it
Having to do things for someone else - again, I am independent and self sufficient and don't expect or need anyone else to do things for me
Others are things that surely would happen even in a FWB arrangement (such as spending less time alone, having less personal space), so why would having to do them in a relationship be any different?
Others don't seem overly onerous to me, though I guess may to some men. I invest emotional energy in my friendships, I would naturally do it in any relationship, it wouldn't be hard and wouldn't be something I wouldn't want to do.
The not having sex with other people is one thing I definitely would expect.
So if those are the "negatives" that a guy thinks will happen if he is in a relationship, and many of them are things that i don't actually expect, it seems I have a different (perhaps non-traditonal) view to many people about what relationship is like/how it would work? I would still call it a relationship, but seems these men who wouldn't want to relinquish these things might not want to call it a that (or to them it wouldn't be a relationship)