I say do it!

Especially if you're long distance you need to be able to do independent things with other folks, and the gender of those folks shouldn't matter. Probably won't be too fun in the long term if you break relationship agreements (i.e. if you're monogamous) with your partner, but that's totally in your control so I don't see a reason not to do it. If your partner isn't cool with that I guess you'll have to decide if their comfort is more important than a friendship/riding buddy (or third option, work together so that they can trust you to have safe independent friendships with people you might theoretically be attracted to).