Does Long-Lasting Parental Marriage Affect Your Relationship?

Potentially, but his or her parents’ relationship and YOUR relationship are two different things. People can have parents with good relationships and be shitty at relationships themselves.

It just depends on what kind of person they are

not really.. because I feel like they would have unhealthy expectations of you or your relationship with your parents.

Not necessarily, I feel like it depends more on who the person I'm dating is, I've dated someone who's parents have been married longer than mine, but he seems to avoid my family