I need advice: Do you have to be married to consider a threesome?

SO MY BF BROUGHT UP THAT HE WANTS A THREESOME AND TOLD ME I COULD CHOOSE THE PERSON (FEMALE), YOU KNOW SOMEONE I'M COMFORTABLE WITH. I'M UP FOR IT SINCE OUR RELATIONSHIP HAS BEEN BASED ON PLEASING EACH OTHER BUT I WAS READING A BLOG ON HERE AND MOST COMMENTS REFERRED TO MARRIED COUPLES, DOES IT REALLY MATTER IF YOU'RE MARRIED OR NOT??

[ED NOTE: Also see "I need advice: He Wants a Threesome"]