Marriage isn't just a nice vow to love each other 'till death do us part. It's the formation of a new household, a legal entity. You can think of it as somewhat like a corporation of two. A lot of tradeoffs and intangibles go into marriage. For example, one person my raise the kids, saving huge amounts on childcare and freeing the other partner to focus on full time work. Who owns how much of the family's assets in this case? For this reason, unless otherwise covered in a prenup, all assets acquired during the marriage are considered marital property and divided as such during divorce.
Don't try to "shield" this from the divorce by giving it back to your parents. Transactions are all recorded and the courts will not look kindly to a big transfer before divorce.
When your parents "provided" some money to invest, by which I assume you mean they gifted it, that was an asset acquired during the marriage. They (or you) may be very unhappy about it getting divided as part of the divorce, but it was a goofy scheme to begin with. A gift is a gift, and so your parents should not expect to recover that money in the future unless it was documented as a loan at the time. And don't try to recast it as a loan after the fact...that's just asking for trouble.
If you do end up getting divorced then do the right thing and divide everything down the middle. The lawyers are the only ones that win if you guys start fighting about this in the courts.