The most obvious issue is rape and more specifically irrevocable consent to sex. The law, prior to R v R, implied legal characteristics of marriage which are peculiar to marriage (i.e. rape applies to everything but marriage; why is this? Are these characteristics still present in non-rape law?). Then there is the issue of changing the law to fix this "problem" (does it cause uncertainty in criminal law if law is changed overnight? Perhaps compare with joint enterprise).
These are only pointers and don't feel the need to tick them all off and think "Ta-da, I have finished the work now!". Some might be useful to you and some might not be.