I decided to research this topic before answering, but much to my surprise this is what the internet spiders found first it seems that we are hated and feared more than I realized obviously non believer relationship must result in good relationships which outlast believer ones and convert believer. There are dozens of these this is but one example: [biblereasons.com]

Good question. I think that people living longer leads us all to more non standard relationships in life. There are quite a few sociological theories which suggest that in pre historical societies people lived more communally. Men went out hunting and women and children farmed and gathered berries and stuff.
So if you look at it like that then the Christian church imposed an artificial moral structure upon us and we were never naturally monogamous.
Me personally, I'm a serial monogomist. I haven't had many relationships and they have tended to be long. I've been single for around 3 years now and although I adore men I could not see me living with one full time again.
I like the dynamic of dating and making an effort to see someone and you lose that when you live together. Most men just are not that domesticated lol and I've got no children, never wanted any, and I deffo don't want any adult male babies to look after!!
Maybe it's just that I've always picked losers but I look after myself and would expect a man to do the same.

In my mind, freethinkers -- people who are not trapped by the dogma -- are more adventurous, more willing to push the envelope beyond society's norms. However, if a person has a desire/want/need for a monogamous, committed relationship, being a freethinker does not necessarily rule that out. I'm amazed and aghast at the number of religious folks who think that our non-belief means we have no morals.

I suspect this is true. Certainly we enjoy the benefit of not being shoehorned into the (very recent, sociologically speaking) expectation that we shoehorn ourselves into a "nuclear family". In my experience, atheists and agnostics are more open-minded generally, which means they are likely to be more open to different solutions to life situations than the ones prescribed by religious dogma.

Monogamy. Which can be held by different configurations of participants.

I think it is the other way around, or at least works in two directions. Persons in nontraditional relationships tend to drift away from and not to be a part of a religious belief system that isn't supportive of them.

I do think persons raise as atheists are less likely to have hang ups about entering into a nontraditional relationship.

However, since sexual orientation is fixed at birth, the number of atheists in the population does not change than number of persons (per capita) who are likely to want to enter into nontraditional relationships.

The only thing that changes, in regards to relationships, with atheism over religious belief, is the level of self acceptance and willingness to be true to oneself.

Your question implies that, because atheists and agnostics have no belief system thrust upon them that indoctrinates in terms of consequences, everything is fair game. But I don't think that's true. I think we each have a built in moral compass that sets our boundaries in all sorts of areas, and relationships is one of them. For example, relationships with those that are unable to make informed decisions because of their age or mental capacity. I could go on

Live and let live. I don't care what you do or how many partners you have. If it makes you happy, go for it. Personally, I don't share.