CharlesDickens - we are living proof that those are not the only two options.
I would probably say that monogamy is the norm, and works for a lot of people. But it's not the only way - and hopefully you've seen that mummy and DP can have had a loving a committed relationship for many years, whilst also having other relationships which have been/are important to us too.
That it's quite a busy way to live and it can be complicated.
And that they should spend their time with people who care for them and are positive influences in their life. And create their lives in ways and shape that work for them and those they love.
And be honest with people, always, and communicate often and well. That some relationships last a lifetime and some don't, that you can't always know at the time, but that it's ok. Relationships change, and sometimes end. And that's ok too.
That the happiness of people in a relationship are more important than the relationship itself.
That sex is good, and good for you provided you do it safely and healthily. That they should enjoy their bodies and delight in all the things their bodies can do.
To choose the person they have children with - if they choose to have children - carefully. Because whatever happens you will be co-parents with that person forever.
By which time they're probably wishing they hadn't asked.