IE (and other similar sites) is great and there are lots of people on there with genuine open marriages in among the trolls and bots.
Just ignore people who say ooh dear you will have an affair, you will fall in love and leave your wife and it'll all be terrible. And for those folks - if you have never had sex with someone other than your spouse, after marriage, then you don't really know all the ways it can go. And equally, if your only experience is the terrible betrayal of an affair, that is naturally very upsetting, but you're not in the same position as the OP, so I wouldn't give advice if I were you.
There is a world of difference between having permission to explore, and doing it in good faith, and the scary, compartmentalised, emotionally stunted, stubborn self delusion of having an affair. Non monogamy is a different process. To some extent it inoculates you against doing something stupid, because you are thinking it through. You are not blowing up your world on a fantasy of perfect love and baby-like attachment to a fantasy Other. You are not running from yourself or from your marriage.
You will meet sensible women who see that there are many colours and kinds of intimacy.
Many of us would like to have exciting, sexy, love, deep companionship, life partnership, shared parenting, intellectual equity, soul mate status, all with the same one person. Who wouldn't? But life does stuff to us and sometimes we can't offer all that to the same person. And splitting out the parts of love often gives a deeper appreciation of the value of each part. I'm not with DH because we are each other's only sexual option. We choose to be close every day and value the whole of the relationship, not only sexual exclusivity.