Depends on the people...I have never known anyone that were asked to convert, including in my family, when my cousin (raised Methodist) married a Jewish guy (whose parents were orthodox)- they had a combined ceremony with a minister and a rabbi (sounds like the beginning of a joke) and didn’t really raise their kids to be any religion and have been married since 1985. I know one person that converted after marriage but that was simply his choice and he wasn’t pressured into it.

Yeah, not a good idea. I can understand people of different denominations of the same religion being together but I never understood why 2 people with such wildly different world views could dream of being compatible long term.

Mormons are even worse. From a very young age they are taught not to even date outside of the faith, to marry young (again, only to other members), and to multiply and replenish the earth. It's for the same reason as Catholics - as others have already said, it's their most reliable recruiting mechanism. I saw a really cool infographic this week that showed population growth by state and broken down by moving in/out and birth/death. Utah was #4 in growth and the large majority of that was from births. They're growing their own clone army <cue Darth Vader music>

Peer pressure?

The institution of marriage is a dying beast. It has become obsolete. Monogamy is great and err'thang, but putting bells and whistles around it doesn't really mean much nowadays - and it never really did, I guess. People grow up with the notion that marriage should be a thing in their lives once they get to a certain level or whatever.
I've had enough dealings with a religious significant other to the point where I don't want any part of that, anymore. Why I'm on here, kind of.
When it comes to loving and being with someone it's about things lasting as long as they can, each person putting the effort they think they need to, and enjoying the company of the other person as much as one can. And that's the best we can hope for from ourselves and each other.
Marriage is just an expensive, obligatory, religion-centric DLC-sized skin for a particular version of that equation. I know it doesn't answer your question, but my aim was to challenge the premise. Who cares about marriage, anyways?
If someone forced me to convert for any reason, I'd peace the fuck out.

It's not the belief in God that is wrong it's the belief that only one road will take you to heaven is the flaw

Churches want to grow, as their future incomes depend on it. Both parents belonging to one religion who raise their children in that religion are the best way to insure the future of that religion.

The primary reason why most churches oppose birth control, is that it reduces their future membership potential and future income. After all, the Bible defines life as beginning with the first breath after birth. So, recent dogmatic reasons about preserving life are not backed up with scripture or doctrine. It is about income tot he church. They use the preservation of life argument to just not seem so crass.

The catholic church in particular are prone to try that tactic on to attempt to maintain control. Most of the more rabid sects tend to just prohibit their members from even associating with nonmembers, so a marriage seldom happens.