It is definitely possible, and I would think almost probable. If you have meaningful relationships with those of the opposite sex, and you find them at all physically attractive, it's difficult not to fall in love.
The problems come into play when one person "in love" wants something different than the person they are "in love" with. I personally would not be able to sustain a close relationship with someone who shared physical intimacy with someone other than me. However, there are lots of people who are fine with "open marriages" and that's ok too. The key is to be with someone who thinks along the same lines as you, and thereby avoid unintentional emotional harm.