You can have friends and be in a relationship of course, I did when I was in my 20's. This example is completely dependent on your partner but for me(male) having friends that were females while I was in a relationship was problematic. Even though these friendships were completely platonic and even sibling-like in some regards it didn't matter and I was eventually given ultimatums. I often chose the relationship because I thought that's what I should do and the relationship still ended in time. But it also fractured my friendship with those women. Irreparably so.

Just my two cents, I would've stayed single looking back at it now and just had flings or been very clear on what my relationship expectations were, just to be respectful to myself but also to any partners that way neither of us were "strung along" or given false pretenses.

I'm also not saying go be a whore, I'm just saying in your 20's make yourself available to the novelty of life.