I enjoyed the film but definitely appreciated it the second time around- with subtitles (there is a subtitled version on youtube). I don't think the portrayal of Monty is where the potential issue of homophobia arises, or that he would necessarily jump any man that moved. Marwood is a very attractive young man The potentially homophobic bit is Marwood's attitude; he's pretty inhospitable to sodomy before he's introduced to Monty.

I also find it odd that everyone assumes Withnail is gay and that Marwood is straight. If we take the film as straight (no pun intended) autobiography then Marwood is straight but if you didn't know it was an autobiography... I think that people also assume that any flamboyant man is gay. Many of my male fellow Drama students were flamboyant and witty and yet straight (although some of them liked the whole idea of having a mysterious sexuality). Withnail is also the only character to make references to women, calling the schoolgirls he passes "scrubbers!" and then telling Marwood that they "love it". Also, when Marwood is confronted with that bull (obvious symbol of Monty), Withnail tells him that "[the bull] wants to have sex with those cows", but Marwood is too distracted by the symbolism.

It's really one of the bizarrest friendships ever, as Withnail basically pimps his friend out to his uncle in order to get booze.
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You cannot have it both ways. A dancer who relies upon the doubtful comforts of human love can never be a great dancer. Never. (The Red Shoes, 1948)