I believe most are involved in a relationship and are merely "looking" (at porn as apposed to the actual thing).

Many couples join our big cocks sites (hetero).
Many single females join our sites.
A boat load of married closet dwellers (married to female but prefer males), or married bi-curious individuals, join our hetero site for the sole purpose of cruising our solo male section, auto-fellatio images and to access our "other" big dick site (gay).

Of course there will always be the single recluse that would rather look at digital individuals rather than trying their hand at pick up something real or live.