Not quite the only. Diners Club USA also has Club Rewards to Marriott Rewards conversions at 1250:1500. Though if you don't already have a Diners Club card, they're currently not taking applications for new accounts (and it's unclear if and when they'll restart).

And if you've got time to wait, Amex MR can transfer into SPG (though poorly, at 1000:333), and SPG points are likely to turn (at some unknown ratio) into Marriott points in the future.