Strength doesn't have as many useful checks as Agility. Being able to smash the Manor door is neat, but Agility is necessary for dodging Aggressive Harpies, escaping Drow snares, stopping Brigand pickpockets and stealing food from the Harpies and Ettin, off the top of my head. Being able to grapple/sex enemies in battle sounds great on paper, but it's not consistent. Some enemies, like Goblins, are notoriously difficult to dominate sexually, while others, like the Fire Elemental, cannot be dominated at all. Know what is consistent? Hitting them until they fall over.
I get that game balance is hard, but the only real reason I see to play Strength over Agility as it stands is for aesthetics/flavor. Not saying someone is wrong for playing the game that way, just saying I wish the game made it more mechanically rewarding.
If you want to go for consistency, just be a slut. Loose every fight and let them fuck you. Pleasure them. You can get past each time. It can be consistent based on your style of play. I would personally level both up and use the better sword being the broadsword if I could. You run the broadsword with both high strength and agility. Given that you can increase both by 5 after creating your character, you are guaranteed better damage by purchasing the broadsword. Your stats all have a cap, and when you can't level up your agility, you level up your strength to deal more damage.
You don't need to stop the harpies, just blow them. You can also get out of drow snares using boots, or by going through the dialogue, you can also talk to the brigand pickpocketer to giving it back. Winning grapples and preventing yourself from getting raped is another perk of strength in terms of grappling. What you deem useful is subjective. Especially the checks.
Your preference is to use one over the other it seems, but one could argue that you shouldn't be using one over the other and instead be using them together. You can deal more damage that way.
The additional content for having higher strength isn't rewarding? You get far more extra scenes with characters for having high strength over high agility. Check out all that it opens up with Urka. There is content for having high strength.
There is more to the game than just the combat. That is why Charisma is important. You open up actions with characters which is tons of fun. All the scenes are entertaining. The point is that you can play the game the way you want and you have the choice to use your perks to aid in that. Your agility means less than strength for someone who wants to see more scenes. That person would value charisma. I personally play with a pretty well rounded character because I want to have the most options. You can pass most checks with an 8. Now given that we can increase just about any stat by 5 during the game, that means the minimum score of 3 for each stat would be sufficient. On top of that the game also provides us with bonus perk points for getting achievements. Now that you have 8 agility, what more do you want to do? Any class can get to it, as well as getting just about all their other stats up to 8. Agility is only so important once you get it high enough. There are plenty of checks so that you can play it different ways that suit you, each having different scenes to get you there. I would know, I try to keep getting every scene to have full completion. The scenes and checks are rewarding if you value that. What you value and how rewarded you feel is up to you.
The choice for agility and strength isn't binary, you can choose both. Nothing is being forced. Only so much agility is useful, so you have plenty to spend on other things as well. I would suggest spending a good deal on strength to reach the same amount of combat damage and still reach those checks for more scenes. There is no reason you should be choosing one over the other unless you are doing it to yourself.