Is it ever ok

It depends on the type of person you are. I say yes to that because as a mother i always put my kids happiness before mine. There have been many occassions over the years ive refrained from doing something that woukd make me happy because the people i love would be unhappy by my actions.
So for me the answer is 100% yes.
But i know plenty of peeps that feel they have to put their own happiness first because they dont feel they would make anyone else happy of they werent happy themselves x

It is , but be careful. There are a lot of people that would take advantage of that !!!!

Sure, I think we do this unconsciously when we love a person.

Yes when in a relationship it happens without thought if it is a true love type relationship, because the other person should be doing the same for you!

This is absolutely what happens once you have a child!

Of course it is...that's the cornerstone of selflessness.
You just gotta be selective and smart about it.
It's not ok if someone else's happiness is going to result in a lifetime of misery for you.

Sometimes its a compromise that is acceptable
Every time its a punishment you put yourself through, becoming second to them.

only if they feed you cake while touching your knees, saying you lost weight

Yes, but you need to put your needs before anyone else's needs. Otherwise you'll be frustrated.

I used to do that, but not anymore. My husband and I are pretty equal in our happiness.

Not if it doesn't make you happy to do so.

No, unless it's your child.