Let´s make something clear: kids alter relationships.
Men, most of them, like to enjoy life without the responsibility. Women in return, marry with the idea of starting a family and have kids because for many women, having kids is a dream come true, starting a family is what we have been taught and in other cases shit just happens. For most women kids become their first love and their love life, their relationship, suffers as a result. One because men do not like to share the attention with screaming little beings that they will need to care for and support for at least 18 years, small beings that they don´t know and that enter their house to offer sleep deprivation, diapers, bottles, expenses, more in law time and endless weekends at home. Not fun. Two because sex becomes less feasible when the kids are in the house. There are of course other instances in which the father becomes in love/obsessed with their offspring. In these cases the woman will get pissed off, depressed, jealous and resentful, that also kills the relationship.
Who likes to add responsibility and more pressure to adult life in these times where job and financial stability are as flaky as an onion? Who likes to wake up naked after a night of passion and lust and see a small walking baby with a pacifier enter the room and crawl into your bed, the bed where you just had sex? Nobody. Besides, women when they become moms will be so preoccupied about the kids that they will not appear as sexy as before not to mention relaxed or done up. They will be one thing instead: exhausted.
Many men affirm not to feel attracted to their partners after they give birth. How can you be attracted to someone that carried and expelled a baby out their vagina in front of you? Do you love that woman? Yes. Do you find her misterious, sexy, dirty in bed as before? Not really. After your child calls your once sex partner mom repeatidly in the middle of the night sex appeal kind of goes away, not to mention the breastfeeding part.
I have been married twice and twice I had kids wiht my respective husbands. We love our kids to death but the kids in both cases, ruined the marriage and the relationship. After that I had a serious boyfriend whom I thought to marry and one again, the kids, the pressure that kids and their education brought to the table killed the relationship. Because let´s make another thing clear: if it is hard with your kids it is even harder with someone else´s.