Ah, I see now that when I say flex I really mean contract. I wasn't talking about making a bulgy biceps muscle but I didn't have another way to say it, except for "activate."
In regards to that, I do think that day to day activity has us using flexion more. When we lift things, when we pull open a heavy door or pull a chair away from a table. The action of extending isn't quite as common, at least not in my daily life except for training.
Relaxing and not using strength in doing aikido to me means not tensing muscles trying make something (technique) happen. If I tense anything, I impede my own movement. Doing ikkyo, for example, I try to relax my arms and even my hands when contacting uke in two places. It makes my movement much more fluid than if I grip hard, which sends tension up my extended arms and into my shoulders.
All muscle movement is caused by one or more muscles contracting -- "activating", as you put it. The question is, what muscles? When a muscle isn't contracted, it's relaxed, which is why "Relax!" is so useless as instruction -- if we really relaxed, we wouldn't move at all!
Then you have flexion and extension. Flexion means to decrease the angle of a joint; extension means to increase it. This is most visible in the extremities, where we speak of opposing muscle pairs such as the biceps and triceps. When the biceps is contracted, that causes the elbow joint to flex; when the triceps is contracted, the elbow joint is extended.
My simple (probably simplistic) take on it is that many (most? nearly all?) aikido waza are based on extension, and that takes a little getting used to. Maybe our day-to-day actions use flexion more? I don't know. But I think that many people, when they think to themselves "make an effort" "move that person" or whatever, their muscle memory tends to fall back on flexion. So, obviously, it's not simply a matter of relaxing ("Relax!") the biceps; you need to contract the triceps, that whatsisname funny muscle on the underside of the forearm, etc. "Extend" is another command you hear a lot, and many people seem to think it's something ethereal (probably because so many people say "extend your ki", whatever that means). But it's not ethereal at all. It's simple body mechanics.