In the middle ages, what was something that was viewed as taboo and severely punishable by the government/clergy (at least officially) but that most people did anyhow? How did people reconcile these contradictions in their minds?

Excellent questions, especially the second one. I wrote a conference paper last year on the tension surrounding lay (in my paper I only discussed 'chivalric' atittudes) and ecclesiastical attitudes to marriage (and adultery). It is quite late here and I have a busy couple of days ahead but I will try and find the time to convert a section for you, if you want.