The OP is a little unclear to me as well, but, if I could dare to oppose the moderator, I do not believe that the 3rd person inflection of the imperative in v6 should really be taken, in this case, as overly mitigating, since contexual elements seem to suggest otherwise.
The Pharisees come in v3 specifically "to test" Jesus, i.e. whatever we read in the ensuing text should not be taken completely at face value, since it is merely a means to a test. The Pharisees then ask if divorce is permissable "for any reason." Notable is that Matt has added that phrase vis a vis Mark, in whom the question is simply "is divorce permisable (at all)". Matt's addition likely reflects his placing of the question in the context of the long running Pharisaical debate as to whether divorce should be allowed only for sexual impropriety (Shammai) or could duvorce be justified on many other different grounds as well (Hillel)? In otherwords, while Mark has Jesus ruling on the issue of divorce itself, Matt is asking Jesus to settle the debate between shammai and hillel. Note that in Matt 5:32 Matt has already had Jesus side with Shammai. In any case, Jesus responds by hearkening back to the creation ideal "from the beginning" in v4 and then a verbatum LXX cite in v5, followed by his own interpretation in v6, which as an expression of the creation ideal, therefore looks like an absolute blanket prohibition on divorce. Or, in other words, though in v9 Matt does have him side with shammai again, the essence of his answer (still preserved in Mark) is "What? Shammai? Hillel? They're both full of it!"
This is more confirmed in v7, where the plot picks up again from v3 as the Pharisees proceed with their testing. "Why then did Moses allow divorce?" It works as a trap because they have, apparently, just tricked Jesus into contradicting Moses. Moses allowed some divorce, but Jesus just made a blanket prohibition! "Ha Ha Ha, what do you say to that Jesus boy!!" The contradiction only occurs if v6 is taken as an absolute prohibition.
The whole scene is yet another expression of the classic gospel motif, that of comparing Jesus to Moses (always to find Jesus superior) and thus to compare nascent christianity with judaism (with similar results). In this case, Jesus demonstrates in v8 not only how they do not even understand Moses properly, but in Jesus own superior understanding of Moses and ethical transcendence (i.e. stating a higher ethic than Moses), the typical Jesuaic superiority is confirmed.
edited to fix a garbled thought.