Before I started dating my now wife, she made comments about being absolutely against gay or bi people... On our first date, she started asking questions about my past. About half way through the date she asked me this question "so... You've told me how many girl's you've had sex with, have you ever had sex with other men?" She then chuckled (as in she was asking as a joke)... I waited a moment, not sure if I should be honest with her... After all, she constantly made comments about how disgusting it was.
After some thought, I decided it was only fair to be honest... There is no reason in my mind I should lie about a part of my life with someone I could end up spending the rest of my life with. So I answered her question, "yes, three"... She waited a moment, then told me she asked as a joke because of how I answered her questions about those I've dated.
The rest of the night I wasn't sure there would be a second date, as she seemed a bit standoffish... But by the end of the night she seemed perfectly fine with my sexuality... And 3 year's later we got married.
We openly discussed what terms to hold our marriage to... We decided (her mostly) that it was unfair to prohibit that part of my sexuality in marriage... So my addition to make thing's a little more fair (or even) was the rule that we were both allowed to venture separately on the condition that if we were leaving the house with the intent on having sex with someone else, we had to tell the other spouse beforehand; in the event we did not leave with that intention, but the moment got either of us there anyway we were to notify the other as soon as possible (not required to stop during foreplay to call home, but needed to call home when finished).
This has worked well for us during our three year's of marriage so far, and hopefully continue to. We have each played apart a few times without jealousy, as well as invited a select few men to join us in our bed together.
In closing, if you can't be honest with your partner's... There is no reason to think they can be honest to you. If you can't remain faithful to your other half, you either need to tell them of your infidelity or end the relationship... It is possible that telling your partner will ultimately end in separation or divorce. That is a consequence of your infidelity to begin with... I think most partner's would be more likely to forgive if you confess versus them finding out from someone else.
Sorry for the long winded reply... Be honest with yourself, be honest with your other half.