Both Soul and Maka are very touch starved, albeit for different reasons. Maka, due to the absence of comfort from her mom and her aversion to men from her dad, probably avoids physical touch from anyone she isn't extremely close with.
Soul, however, growing up in a restrictive and cold household, probably never experienced much physical affection from anyone after his toddler years. He probably lives by the "you can't miss what you didn't have" mentality.
The beginning of their partnership is probably a bit awkward and stiff as they start to get comfortable with each other. Touch between them starts and stops at handshakes and whatever's necessary for their weapon/meister training. This starts to expand in to other areas slowly as time goes by and their trust and comfort with each other starts to deepen.
A while into their partnership (about the time of the series), you can see the lines and boundaries start to blur. There's trust and safety present. For Maka, Soul becomes one of (if not the person) the people she trusts the most, someone she knows will carry her miles on his back with no hesitation. For Soul, Maka is a place of security where he's allowed to exist as himself with no expectations nor indifference. They dance together (mostly within the black room), they push, pull, hug, jab with each other like it's second nature. And honestly, at that point, it probably is.
After they start dating, they don't ever become a couple big on PDA. It doesn't seem in their nature. Maka is pretty strict about boundaries and Soul has always been a private person. But, they have their cute, secretive moments. Soul will tap out mindless medleys on Maka's thighs when they sit next to each other and Maka, used to doing things with her hands, will almost unconsciously play with Soul's fingers almost like a fidget toy.
When they're at home, they're always touching each other in some capacity. Back hugs while one is cooking, tangled together on the couch while they're each doing their own separate hobbies (i.e. reading and video games). It takes awhile for them to move into one bedroom, and for years they just both end up in the bed of the person who goes to sleep first (usually Maka). Touch now comes as easy as breathing and their intimacy levels are super high. Kisses under covers, legs over laps. They're a super affectionate couple.
