This probably sounds obvious, because it follows that if millennials aren't getting married (or even entering relationships) then of course they're not going to be going on a Honeymoon. That on its own is having an effect on companies which offer honeymoon packages, but there's more to it than that. Even within that small percentage of millennials who do choose to tie the knot, many of them are deciding not to go on a Honeymoon after the event.
The reasons behind this are quite depressing. A lot of newlyweds simply can't get enough time away from work to take a Honeymoon. Because many millennials are self-employed, not working means not receiving a salary, and having just paid for a wedding, their salaries are needed more than ever. Some apparently even choose to take 'unimoons' - a vacation away from their partner from a week to compensate for all the stress that went into planning the wedding together.