No one is saying your relationships aren't important.
But you have to think very carefully before you marry/have kids, whether this person is a good choice to be the future parent of your children.
Sometimes we get so caught up in "winning" the relationship, we don't look ahead.
I speak from experience. I met my H in our 20's. I was crazy about him, and I was hungry for a relationship to validate me. I also, like many women in their late 20's, wanted to start a family before I was in my 30's.
I blew past many red flags in his behavior because I was so in love with him. He slept with an old girlfriend the night before our wedding and spent the first six months of our marriage writing to her (I didn't find out he actually slept with her until years later, when I was 9 month pregnant with our second child.)
I attempted to kick him out when I found out he'd been writing to her, but he pleaded and I let him stay. Eventually we renewed our vows privately and I thought things were good. We had three kids, a good life, a good relationship - except for his periodic bouts of the blues and dissatisfaction.
He had an affair when the kids were preteens and early teens. Devastated them and me. And I felt guilty that I had taken a chance on him, knowing he was capable of cheating, and now my kids were paying the price in pain.
We reconciled with the help of this message board, and we actually had some of the best years of our marriage. Unfortunately, a combination of health issues and MLC finally led to my H leaving. My life now as a single woman is actually really great, but it still pains me that my kids have to suffer - and they do, even though they are all in college now.
My point in telling this story is this: if you are young and early in a relationship, and an affair is happening, the odds that the behavior will repeat are very very high. And when there are children involved later - it is heartbreaking. Ask yourself why you don't think you deserve someone who adores you and treats you RIGHT?