My gay couple friends have been together since the dawn of time... but every now and then, they take separate "vacations." It seems to work, and they have a solid, great relationship. But maybe that's because men in general can separate sex and love better than women.
Although, if I were married to someone for a very long time, say twenty years, I just don't think I would dump the relationship for an indiscretion. Besides, after all that time, it isn't as though hordes would be lining up to fill the dance card. Lol. However, if it happened after a year of being together, the signs would seem to indicate that the other person isn't trustworthy or "needs" too much.
My motto is, "keep little hands and minds busy." It's the best deterrent there is.