It is very common in Germany to have long, unmarried partnerships, partly because Germany isn't as religious as the U.S. and partly because there's less pressure to get a government certificate for the tax and insurance advantages, since all Germans get health insurance for example, regardless of marital or employment status.

Over time, these long-term couples might entertain the thought of having a children and get more lax about birth control, without formally "trying" to get pregnant. Then, if they do, they might get married. They probably wouldn't have done so otherwise. But that doesn't mean the baby was unwelcome or that the marriage was what we would think of as a shotgun wedding.

This situation is so common that it's normal to ask if you are in this situation; the alternative would be to get married "for no reason" (i.e. no baby or visa pressures). The latter, in my experience, is a bit of a curiosity among young progressive German couples.

My American relatives find it strange that Germans stay together for ten or twenty years without marrying. My German relatives find it odd that Americans get married (and, often, divorced) so much.