Any person who sacrifices his life for an innocent is causing others to suffer, provided he has family: Parents who'll have to deal with losing a child, a spouse who will have to deal with losing a husband or wife, children who will grow up sans a mother/father.
It's not the same thing at all. He's not just giving up his life. He's effectively giving up MJ's as well.
That potential future daughter will never be born, but I don't see undoing a potential child as particularly immoral.
How could erasing a future child from existence through a deal with the devil possibly be considered moral?
As for Aunt May's comments, like I said, I don't see much difference between that and "Save yourself! I'm not worth it!" Your typical superhero is not the type to not treat a life as precious just because the person's really old.
Yeah, I'm not seeing that at all. May had accepted her fate and wanted to be reunited with Ben again. If Aunt May KNEW that Peter was giving away his marriage with MJ as well as his future child so May could have a few more years of old age, do you seriously think she'd approve? She'd be heartbroken and would probably resent Peter for it.