I've been sitting on this one for a while because, well, you'll see.....
Drink water from your own cistern,
running water from your own well.
16 Should your springs overflow in the streets,
your streams of water in the public squares?
17 Let them be yours alone,
never to be shared with strangers.
18 May your fountain be blessed,
and may you rejoice in the wife of your youth.
19 A loving doe, a graceful deer—
may her breasts satisfy you always,
may you ever be intoxicated with her love.
20 Why, my son, be intoxicated with another man’s wife?
Why embrace the bosom of a wayward woman?
It always catches me off guard when I'm reading the Bible and things get overtly sexual. It doesn't bother me, quite the opposite, it reminds me that sex is good, and should be discussed by people who follow Christ, because it is discussed, quite openly in the scriptures. I think th reason we don't discuss sex more openly in the Christian community is because we mistakenly confuse Victorian era shame about sex with scriptural teachings. More plainly, we think the Bible says not to discuss sexuality or sexual things, when it doesn't. This has caused the church to avoid discussing sexual matters, and has lead to it's lack of relevance in such area's. The good news is, I think we are moving beyond that and the Chrisitian community is beginning to realize how important it is for us to engage in good, honest, forthright dialogues about sex and sexualtiy.
As for this passage. Why would you need to cheat on your wife if you're enjoying her? The answer is you wouldn't, so go enjoy you're wife! Translation, have sex and plenty of it, while you're young and you can. Or at least that's my common sense interpretation.
PS - the Proverbs are littered with references of how cheating on your wife will wreck your life, and basically every movie and book I've ever seen or read that contains adultery in it always ends extremely badly for the adulterers. It's such a universal truth that even people who don't believe in the anything recognize the destruction that infidelity brings upon a life, a relationship and a society.