The plant looks fine and healthy. To keep it upright use a stake, or tie the leaf stems together loosely like in a bouquet. Be careful to not rub any new growth (or stem that will produce new growth) against the tie or other leaves otherwise the new leaves would have scars.

Those aren't stems, they're petioles. They attach the heart-shaped leaves to the actual stems, which you can see towards the base of the plant. Monstera deliciosa is a vining plant that can't grow upright without some additional support. Otherwise, it's just going to flop over and grow outwards, just like yours appears to be doing.

Yeah, I know. My comment wasn't directed at you. The OP referred to them as stems in their post, so that's why I brought it up. As for growing Monsteras, I'm never concerned about the direction with which the petioles are growing, just as long as the actual stems are growing upright. Because I've noticed that their stems aren't the most flexible. Once they've hardened off, then that's pretty much how they're going to grow. So I only concern myself with getting the stems growing how I want them, and just let the petioles/leaves naturally spread out from there.

Nice plant. When the plant is younger and the stems only a few inches, it is the petioles growing horizontal that make the plant unseemly like a mop head, besides the spread occupying more than its fare share of horizontal space.

It's because you need to give it something to climb they climb stuff naturally thus why you have long searching aireal roots

Moss poles are easiest. And if you rotate your plant that can help because the plant likes to stretch leaves toward a darker area hoping that they're being shaded by a tree thus they're reaching toward the dark because once again they want to start climbing stuff. I know I didn't use a single technical term I'm so sick I'm surprised I can spell at a 5th grade level. 😅 I I'd definitely give that baby a moss pole and I agree with the person above about making sure your stems are where you want them before they harden I mean the petiole. But the same goes for the stem like once a new one props up all noodly I start pushing it around a bit trying to give it room to grow I have two leaves that are like the exact same size and want to occupy the same space it trips me out all the time because you're like whoa I'm seeing double oh wait there ARE two there. Which in reality they both need sun so I've been trying to get them to stay out from under eachother and it wasn't working so I shoved a naked ivy stem down into the pot and had it come up between them. I'm unsure if this has been successful or not I'm stubbornly leaving it there until they're forced to make a decision. Only one shall live..... I really am sick. Good luck 👍

Here are the twins as you can see it kinda worked. Lol 😆

You can totally tell which one was ontop for most of it's life it's a darker color as well as much thicker. They don't look much like twins now