She only directly tried bearing his children in the episode she surprised him in - Issei's freaked-out reaction was summed up as (paraphrased): "Do you understand what you're saying!? You wanna make a baby!? That's insane! That's insane! That's insane!". And not only that, this attempt indirectly caused agony to Issei, due to the beating that resulted from the misunderstanding (something she certainly wouldn't seem to be all for). So as a result, she likely decided to put the baby-making on hold for now until Issei said/showed it was okay - hence the condoms, and the 'we can practice anytime' aspect. Becoming a Devil also means she's got plenty of time to wait now, since most live 'almost forever'
A lot of comments are made about how Issei is the weakest of the Dragon Emperor hosts at the start and a while into the series, and a lot of observations are made that he is strongest when he is both determined and horny. It might not have been clearly outlied, but there may be a simple enough explanation: Koneko commented that a lot of dragon hosts tend to go crazy with their power and become menaces while Issei isn't as hung up on power. He is essentially powered by sexual lust instead of battle or power lust.
It comes off as a funny joke that Akeno suddenly gains a massive boost in power once Issei promises to go on a date with her, but it actually makes sense. She is a devil a creature of desire the thought of having her desire fulfilled allowed her to draw on that for strength. Same with Rias the thought of what is 'hers' being taken away infuriates her, remember what she said when Issei almost died. 'how dare you take away what's mine.' not the man I love or my friend but what's 'Mine' even if she does care for them it is subtle reminders like that, that remind you they aren't humans but devils.
Sirzechs in the last episode of the first season of the anime seems content that Issei took Rias away from the engagement ceremony and even Grayfia notes that he seems happy about the fact that it's "unfortunate" that the Gremory and the Phenex clan will not have any marriage whatsoever. The reason why he's happy about it is because it reminded him of his and Grayfia's circumstance when he married Grayfia, one of the servants of the Old Lucifer and at that time, it was a big no-no.
Vali in the eleventh episode of NeW did not make his Divine Dividing appear to block Gasper's time stopping ability while he lampshaded that he and Issei were able to block it using their dragon powers (Issei had to make his gauntlet appear). The reason? He did not tap into Albion's power, but instead used the magic reserves he inherited from the original Lucifer. Becomes something of Foreshadowing, as later he was revealed to be a direct descendant (or more accurately, the great grandson)
Further proof is that the Holy Swords and the Boosted Gear all manifested once the time stopping field went off, notice how Vali's Divine Dividing didn't?
It's not usually noticeable since she usually wears her hair in a ponytail, but when Akeno appeared for her date with Issei, her hair is down. Aside from bangs and hair color* Both appear to be black with different tint colors., she looks a bit like Raynare. Also has a bit of Fridge Horror due to her fear of revealing herself as a Fallen Angel to Issei and him hating her for being one.
Issei Does Not Know His Own Strength. Normally this can be shrugged off as Heroic Self-Deprecation, but during the Mid-Class exam he attacked another low class devil (who may or may not have had the power to become mid-class) in Balance Breaker. If he had boosted at all, he would have blown the other person, an ally, in two.
Remember how Kokabiel destroyed some of the school buildings during the fight with him? That doesn't seem too terrifying on it's own until you remember that the OCR building could have been one of them... and Gasper was still locked in the forbidden room.
It gets worse when you remember he's actually allowed to leave the room at night, but chooses not to.
The Evil Pieces system in general. You're forced to give up your humanity and enslave yourself to another devil for the rest of your long, long life. As a Devil, you get to do that to others, who do the same to others...while Issei and the rest of the Gremory peerage may not mind, having a kind and loving master, it's been shown that they're extremely lucky in that regard, as not all devil masters are like her. They could have ended up the eternal servants of someone like Diodora...
And then you remember Diodora was planning to do this to Asia, had Issei not met her on that fateful day...
It's an actual plot point in various later volumes where sacred gears users who are given unfair or deceptive contracts take the opportunity to rebel when their masters find themselves on the wrong side of battles, and then wind up arrested and, surprisingly, are given fair hearings before their sentences are carried out.
CaoCao implants himself with a Medusa's eye to replace the one Issei shot out. What happened to the Gorgon?
Considering he was already Crazy-Prepared, just what did he do to get one of Medusa's eyes?