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: