I don’t think I assume any sexuality on fluther. It’s impossible to tell based on writing samples. They have to announce their preference before I feel like I know anything. Mostly, though, I don’t find myself wondering about it. It doesn’t really matter until it matters.
In meat space, sexual preference is something I assess on a case by case basis, just as I asses education, income, creativity and all the other things I’m interested in when I meet someone. I do assume that they are as they look and present themselves, so if they seem male, I assume they are male, even though I can’t see their sexual organs.
If someone presents as gay, I tend to assume I’m right, but it rarely matters, so it’s not like I have to ask. If it’s important, I’ll be informed, the same as any other attribute of the person. I assess each person I interact with individually, based on my agenda with them. I try not to assume anything—although if someone appears to be one way or another (a race, a clothes horse, erudite, or whatever), I do assume that’s who they are until informed otherwise.