This absolutely was for tax reasons.
DSP rushed to marry khet because the deadline to file your taxes is april 15th and filing jointly as married would offer a major tax break for DSP, as khets likely meager income (someone with a criminal record and no skills likely earns minimum wage working retail) would be combined with DSP's for tax purposes, and since he's married, his tax bracket basically doubles now which gives him a fuckload more leeway in terms of any future tax filings. He likely got bumped down a tax bracket solely because he did this marriage, which likely saves him thousands of dollars ever year from here on out for federal taxes. The one big thing here is the event of a divorce. Activist judges have been throwing out prenups at record rates the past 20 years or so (as if DSP was even smart enough to get one), and if khet ever were to file for divorce, it would absolutely destroy DSP financially to the likes of which we've never seen. You have to remember, DSP makes amazing money, his big issue was he has taken on the debt of someone who makes 7 figures, even though he barely makes 6. If khet were to take half of DSP's assets, he would lose the house guaranteed as well as basically be fucked for a very long time with alimony. Even moving to an apartment in seattle wouldn't be sustainable financially in the event of that happening, and he would essentially have to move back to connecticut or become homeless. Getting married is essentially the ultimate high risk/high reward prospect for phil. If he stays married forever, he reaps major tax benefits for the rest of his life, but if it ends in divorce his life really is destroyed forever.