That entirely depends on the relationship and the couple. If for example, you are both clear that you do not desire each other sexually, then sex as friends can be fun and easy, but not overly emotional. Also, if you both sit down and decide that you want to be together, but that you might at some point have other lovers then I guess it is okay. It's not ideal, of course, unless both parties are very mature about things and talk them through very, very honestly.
I think, for example, if the couple is able to accept the concept of poly-amour, then it is more feasible. For example, you love your partner as a dear friend, as the person you want to raise children with, but not necessarily sexually, and so for sex, you might choose someone else, but that doesn't lessen or cheapen your love for the person who is your legal spouse.
Does this make sense ??? It's all about the way you view love, trust and honesty.