What does make me stand up a little straighter mean?

[SueShe]

"... older men are better at sex than older men..."

How older than the older must the older men be?

That inappropriate comment of yours probably boosted his ego since you posted some very direct message that cannot be misinterpreted.

Now, you lewd comment could be interpreted two different ways but since you could not be more direct in your comment, this is the one I believe he was meaning

1. He was referring to the reaction of his penis to your inadequate post
2. He felt proud to be the target of that comment and he stood up straighter than he would normally do. He probably slouches a little when standing, like a great number of older people with a general tendency for a bad posture.

Whatever the reply, you got yourself a reputation, even if you apologized.

[Anonymous]

He might be talking about an erection or he might be saying that your statement made him proud of who he is

[Avicenna]

It boosted his confidence

[In_Trance]

No idea; ask him what it means