I don't think you can even assume that the young man is the same one. Why does he have to be? There could be different young men, and they don't have to be based on Shakespeare's own life at all.
That's the million dollar question! You will find critics of both schools with some very convincing evidence "proving" both ideas. That's the beauty of literature...as long as you can back it up with textual evidence, you're in like a hair in a biscuit! :) The toughest part about critiquing the sonnets is that no one knows for sure in what order they were written, so they are all isolated from one another...not one, long, conclusive piece.
Critics have wrestled with this question through the centuries, trying to find out the identity of the young man. However, it is important to remember that Shakespeare wasn't necessarily writing an autobiographical series of sonnets and whilst many have argued that these sonnets represent proof of Shakespeare's homosexuality, it is important to remember that sexuality was viewed in a very different way in his context than it is in today's world.
A close reading of the Sonnets is bound to engage with the nature of the relationship between the speaker and the young man. Although the literal truth of the matter can't be established one way or the other, that's not the issue here. What is key is what the poems reveal or suggest about the relationship/s, both within individual sonnets and across the whole sequence (there is a kind of narrative here). Sonnets 1-17, the so-called procreation sequence, have something of the air of being written by commission; there is no sense of a deeply-felt relationship at all. Things change with the wonderful sonnet 18, and sonnet 20 is remarkable as a transition piece which can certainly be read as signalling a physical relationship - but may equally reflect an enjoyment of bawdy puns!
For a provocative consideration of one side of the argument, do read Don Paterson's commentary:
While it is interesting to speculate, I always wonder how it is possible to know the truth of things like this--with so little evidence and so much distance between now and then--with any certainty. Some things are worth debating; to me, this is not one of them.
I think post #2 is completely right -- there is no way to know. That said, you should now read the sonnet sequence for the multiple feelings he has and the ebbs and flows of the relationship. The early sonnets(1-17)praise the young man rather generically and tell him to go have children. The next group (18- 60/70)sonnets show a bond between the young man and the speaker. The next group (70-120~) show his frustration and a generally cynical side of the speaker. The last group are about the dark haired lady. My numbering is very rough, but you can research the sonnets and read for yourself the complexity of the relationship between the speaker and the young man.
Only Shakespeare knew for sure. The same sonnets which many people believe referred to homosexual love are identified as intense friendship by others. The "Dark Lady" sonnets attest to his possible infidelities with a woman, but any other sexual speculation is only conjecture.