Only when the assumption is based on anything besides simple ratios.
There are huge multiples of straight people in the world for every gay person. Statistically, you're much more likely to meet a straight person than a homosexual.
You're playing the averages. This is fine so long as you're willing to accept that you'll find exceptions eventually.
If you assume a person is straight because they don't conform to some stereotype of 'gay' on first meeting, then yes, it's wrong to assume someone is straight.
Intention is almost as important as action here.
There are huge multiples of straight people in the world for every gay person. Statistically, you're much more likely to meet a straight person than a homosexual.
You're playing the averages. This is fine so long as you're willing to accept that you'll find exceptions eventually.
If you assume a person is straight because they don't conform to some stereotype of 'gay' on first meeting, then yes, it's wrong to assume someone is straight.
Intention is almost as important as action here.