After watching 1 + ½ Seasons of K-On! I have come to the following conclusions about the members of Houkago Tea Time.
Yui and Mio are in a lesbian relationship together and love and support one another as they tackle the microphone equally during gigs.
Ritsu is currently questioning her sexuality but I believe she is a closeted pansexual with a preference to women.
Tsumugi is a confident lesbian with a heavy crush on their teacher, Ms Yamanaka (this is even heavily hinted at in an Episode during Season 1). Tsumugi does not see age as a compromising factor in a relationship as long as both partners are very happy and consenting. I also believe Tsumugi could be polyamorous and while she is homosexual, she may very well be panromantic or biromantic.
Azuza is a homoromantic asexual who prefers to bond with girlfriends through music and academia, but she has a playful side herself.