Europe is very different, and I'm American... For myself, as a woman, I happily co-habitated with my significant other for 5 years. But, when we started thinking about kids I wanted to be married and raise my kid or kids as part of a family. Pretty important to me and someone who wasn't cool with that would have been a huge flag. You may see it as just a piece of paper, but for me it was emotional, symbolic, and yes legal security to have the green light to proceed in both getting a house together as well as have a family. Tax benefits would not have been enough for me to forgo that and yes we did have to pay more in taxes after getting married.
I am now divorced (out of my control) and my kids are a pre teen, teen. If I ended up co-habitating with someone in the future the marriage vs no marriage thing would not be such a big deal, since we would not be raising kids together.
Europe I understand is different both in importance of marriage, and open/closed marriages, possibly the stigma of being an illegitimate child, so what I would do, it's really between you and your significant other as long as everyone was comfortable with the situation (and it was legal).