[RoyalHenOil]

In my experience, being in a stable monogamous relationship makes it easier to be friends with the opposite sex because it reinforces that it is strictly platonic.

It works especially well if they are also in relationships and if your partner is also in your social circle. Couples hanging out with other couples is a really strong basis for friendship.

[lazide]

The last part is key IMO - there are communities where the activity involves spending solitary time alone with the other party, and that tends to be much harder to do stably with opposite sex partners without some kind of sexual relationship forming, regardless of other relationships.