Like most other things regarding this show, the romance isn't worth taking seriously, especially with the turtles. Aside from maybe Leo/Karai, most turtle romances here are intended for comedic value at best.
Casey is obviously April's "real" love interest while Renet and Mona Lisa probably won't show up in that many episodes. Karai with Leo... It might be the closest to one of the Turtles having a serious romance but I get the feeling it won't go anywhere.
I sort of hate to bring up the IDW comics again but if you want something akin to romance, go read that instead of watching this cartoon. There isn't much romance or shipping but characters are shown to form relationships have been given romantic backgrounds. Hell there's even two lesbians in there!
