I don't believe there is any evidence that Frost intended his poem to have any sexual meaning. Throughout his life, Frost always viewed over-explication of his poetry in a very negative light. He spoke out about this many times. See the link below, scroll down to read the comments by George Montiero.

That said, however, as you know, the beauty of poetry is that it speaks different things to different people. And the poem could be interpreted this way if one is in that frame of mind. Psychoanalysts tend to view many things in terms of what people are really thinking in the deep, dark recesses of their minds, so perhaps Frost did have some sort of underlying psychological angst that came out in his writing. From what I have learned and from what I could find, however, there is no definitive proof that Frost himself intended for any sexual interpretation to be made regarding Birches.