I don't assume either.
A few years ago I spent a lot of time with a female friend of mine, we would see each other most days and used to go to the gym together usually three times a week.
A few people questioned whether we were a couple, suspiciously enough all male haha and including one of my neighbours and my driving instructor at the time.
We did have suspicions at the time that especially the guys at the gym might have thought we were together and were tempted to get some tongue in cheek gym T Shirts printed up with "We are" on one and "not together on the other or something like that.
A year or so later I was at a local expo and ended up chatting to one of the guys who used to go to the gym and my friend had lusted after from across the treadmills.
He asked me where my partner was! I don't tend to go for the bodybuilder type of look but my friend does and I don't think it helped her pulling potential.
My driving instructor was slighly more blatent (we got on very well) and, when I confirmed we weren't he was keen to ask whether we had ever considered it etc... men lol :)