Any scientific studies I have seen have not showed spanking as such a bright idea. But of course that doesn't matter to most of those people.
What is considered abusive is matter of opinion and culture. Unfortunately I have to say we did cross a line not only from our own cultural point of view (and we are more sensitive in this topic than most) but from most points of view. When your kid needs a doctor because you were trying to punish him, you absolutely know you have gone too far (it was an accident, but the accident that wouldn't had happen if husband wasn't trying to spank difficult child.) And difficult child did have marks from spankings before that last one. Didn't change a thing in his behaviour. Could had gotten us in major trouble with CPS and police. Did make things worse with difficult child. So not worth it.
You have already spanked your child hard enough to leave a mark and that didn't help. What makes anyone think that hitting little harder and leaving more marks would help? And beating a child black and blue is probably considered abusive also by those friends of yours, who don't think leaving little bit marks is that bad? I honestly don't believe that hitting little bit harder would help. When spanking works, it is usually because it is a clear sign of parental disapproval, not because of the pain. After all, kids hurt themselves worse than any sane person would spank quite often and that doesn't stop them of doing those things. My kids have broken bones and not learn a thing, but have been back to do the same things right after the cast is off. How many kids do you know who have for example fallen with bike, hurt themselves and never rode a bike again? I don't know any.
I had to listen muttering about how difficult child would just need a good spanking quite often. if I could, I avoided the situations, if not, i just listened and didn't comment. You are not raising your child to please your friends. And by the way, if they do often spank enough to leave the marks, and say it would help with your child, you can always ask, how would it be, when it is not helping that much with their kids. If it would be so effective, they would had needed to do it only once.