I have been the OW, years ago at 17 years old with my first boyfriend, a 29 year old married man. He didn't tell me he was married - in fact he lied and said he was separated and only hadn't sold the marital home because of negative equity.
I fell totally in love with him and by the time he told me he was still married, I had been introduced to his family as his girlfriend and it was all very confusing. His wife found out about us, obviously as he wasn't really trying very hard to hide it, and was devastated.
He left her and moved in with me to a flat, then started to abuse me. Physically, verbally, emotionally, financially and sexually. His wife got revenge by having divorce papers served citing me as co-respondent on my 18th birthday at my parents house. We got engaged. He was still beating and raping me, and controlling every aspect of my life behind closed doors. Wouldn't let me take up my place at university, made me have an abortion, isolated me from all my friends.
He left me, two years later, for a girl who had been in my class at school. They are still together and married now but he is frequently unfaithful to her. He won't let her have children and has brainwashed her into thinking she doesn't want them, when I know from mutual friends that until she met him she had always planned to have kids.
I'm not saying that this will be your experience. I really hope it is not. But be careful, and make sure you have a support system outside your relationship.