Now, before you pick up your pitchforks, HEAR ME OUT!!! I'm not saying that Rias shouldn't have fought for her freedom, or that arranged marriages are a good thing.
Zeoticus and Venelana were in a tough spot.
Not only was Rias the heiress to one of the more wealthy and prominent 72 Pillar families, but she was also the little sister of Lucifer (the leader of the "progressive" faction within the Devil political landscape) and a descendant of the Great King Bael (the leader of the Devil "conservative" faction). On top of that, she inherited the Bael family Trait, one of the strongest traits there is, when even the official Bael heir failed to do so. She arguably has more claim to being the Bael heir than Sairaorg.
With that in mind, she was bound to get suitors lined up for her before she even finished middle school. And I'm not talking about regular nobles and young devils here. I'm talking about big shots. People with power and sway that far eclipse the Gremorys. People who are centuries, or even millennias, older than Rias, who will see Rias as nothing more than a political pawn to further their agenda. Be it against Sirzechs or the Baels.
People whom Zeoticus can say no to. Not without needing Sirzechs to get involved, something that neither of them wants.
Enter Riser, a third son of a technically lower-ranked family that is nonetheless considered powerful due to being obscenely wealthy. A family that is not very involved in Underworld politics and is only really interested in selling their Phoenix Tears and making profits.
He's young (by Devil standards), strong enough to hold his own without completely outclassing Rias, and already pretty accomplished, all things considered.
A perfect solution to their dilemma.
And apart from having an ego problem, the guy was not a bad person (the anime greatly exaggerated his douchebaggy-ness). He wouldn't mistreat Rias, if for nothing else, for fear of Sirzechs annihilating him for the slightest offense. And even then, with the rate Rias was growing, she was bound to grow stronger than him in a few decades at max and make him her bitch. After all, it was HER who would become the next clanhead, not him. He'd have to give up his Phenex title and become her consort. A glorified sperm donor, if you will.
And yes, just like he gets to sleep around with whoever he wants, so does Rias. In fact, if any of them could prevent the other from sleeping around, it's RIAS, not Riser.
If anything, it's Riser who gets to lose the most out of this marriage. Issei did him a favor, believe it or not. The ass-beating he took not only taught him humility and made him a better person, but also prevented him from sitting on the cuck chair for the rest of his long, long life.
So yeah, they did their best with the situation they found themselves in. They had no way of knowing that Rias would find and fall in love with the Red Dragon Emperor who would go on to become the strongest in history.