This is certainly one of the more difficult Gospel passages to understand, even though it seems blindingly clear on the surface.  What are we to make of the various ways in which the text seems to be softened?  Someone has apparently added "lewd conduct is a separate case" after the evangelist wrote, the apostles hint that divorce is expected to be a normal part of marriage, and Jesus's words at the end are not on the subject of marriage at all.

Maybe none of this is not even really a matter of sexuality.  Maybe Jesus is actually talking about how we live our lives in terms of fidelity.  I suspect, for example, that he means that we should let our "yes" truly be "YES!" especially when we are dealing with something which affects both us and another person as intimately and profoundly as marriage does.  For Jesus, fidelity is not a matter of living up to the letter of the law, it is a most generous, open, and loving spending of one's self for the other, something modeled for us by both the Father and Jesus.  Those who enter marriage with an acceptance of the possibility of divorce are already holding back their gift of self, limiting even the very idea of that self which they are only half-heartedly giving.  Entering marriage in such a way is more a matter of seeking what one can get out of it rather than how well one can serve the beloved.

I think that Jesus' comments at the end of this Gospel passage are in the same vein: our lives are (or should be) "for the sake of the kingdom of God," and that means some sort of sacrifice (although that is not a word that true lovers believe applies to anything they are or do).  More importantly, this is a matter of counsel rather than of precept: Jesus says "let anyone accept this teaching who can." 

None of us can live his invitation fully, but all of us can aspire to that selflessness which gives all to God and others and which receives in return God, one's own true self, and everything else besides.