Any change at this time in tax law would run ahead of general law where the non-recognition of cohabitation is the norm e.g. in relation to succession rights. The impetus for change in the past was at a time when divorce was prohibited and there was, accordingly, a prohibition on persons entering a second marriage while their first spouse was still alive.
Divorce is now available so that difficulty no longer exists.
To the extent that the social welfare code recognises cohabitation it does so in order prevent cohabiting couples getting better treatment than married couples. The tax code also recognises cohabitation in the same way in relation to the single parent allowance. If it is considered that tax law should positively recognise cohabitation, then the issues of definition and Constitutional constraints will have to be taken into account. Also, solutions which will work or are appropriate in relation to income tax may not transfer across to capital taxation and vice versa.