As a few others have said, yes it is wrong to presume.
If I'm interviewing candidates for a job and I know from past experience that most women don't have enough experience for the job, does that make it right to presume any woman applying can't be a good enough candidate, so dismiss her automatically?
Of course not. I can gain precise information easily just by asking and nothing is gained by presuming either way.
The same is true of someone sexuality, you gain nothing by presuming either way and, if you really must know, you can gain the information easily by asking, so why presume at all? If you presume, you're right 90% of the time, but if you don't presume at all, you're right 100%.
Yes, in life there are times when you have to make presumptions, otherwise things won't work out very well but this isn't one of those times. This is a time when the presumption is nothing but your own arrogance.