You also have to marry something. Maybe there's a pattern, or maybe poetry does not always follow the same grammatical specificity as prose, or maybe both.
aside: I realize i also kind of applied artistic licence in my comment here :), by not capitalizing the first words of my sentences, partly because feeling a bit lazy (on my mobile, typing is tedious and slow enough, even without switching case), and partly because I think single case (no concept of upper and lower case) is a better option.
sanskrit, for example, does not have upper and lower case, just one case, or maybe none.
That's not an issue of the verb being obligatorily transitive, since it isn't. You really should look at some usage examples; this isn't a difficult or tricky question.
The problem in your sentence is that the plain form of a verb is used for states, not activities, and marrying is an activity.
"I am marrying today" is just as intransitive, but without the artificially induced grammatical error.
That's not a valid sentence, unless the person you are to wed is named "Today". Such is stuff as Abbott and Costello cashed in on.
Of course, taken in context there may be implied antecedents in spoken dialog. Spoken dialog is often incomplete and rife with implied antecedents and frequently does not follow the rules of grammer. Sentence fragments and one-word sentences can about because of that.
It is valid and maybe easier to see with a comma.
“I am marrying, today.”
“Today, I will be marrying.”
It is valid in the context of “i will be engaging in marriage, today (“today, i am entering into a covenant of marriage”)” but it wouldn’t be something that would be heard very often. it’s a more archaic usage.
That wouldn't work at all; "regret" doesn't have an intransitive use. You have to be regretting something.