Huh, yeah - I can see were you are coming from. I too think that Issei saves the girls a tiny bit too often. Especially for "the weakest host in history".
However I don't think that Akeno shows her weak side because she is female. That is one strong point of DxD: the characters are thought out characters first and their gender second.
Issei shows his weakness nearly every time. He cries and nags and pouts and begs. He is very open about his emotions, about what he wants and what he fears.
And he is very dependant on his friends. He often needs some encouragement from Rias to power up and he is always worried that Koneko might hate him or what will happen if his desires run wild with Asia. And he clearly does not think that he is worthy to be anywhere near Rias or in a relationship with her.
However he is not strong enough to talk about it. Or is it strength to not talk about it? The story sure does not make it seem this way because only problems grow from Issei's habit to keep his deeply personal worries hidden.
And this might be the cause for many of the relationships feeling a little one sided - the girls establish meaningful bonds but he does not...
Kiba has also shown us his weak side this season. And he solemnly pledge loyalty not only to Rias but later on to Issei. To me it seemed like he was deeply afraid to lose his friend over what he did even though Issei did not see it as a big deal.
Sounds familiar? It's similar to the thing Akeno did this episode.
And Gasper... well, it looks like his gender was more of a coin flip decision than anything. He is too whiny for either gender but it is maybe for the best that he is not female...
About the other girl you mentioned: I have never seen anything of the Monogatari series but I know To-Love-Ru.
And Momo is a perfect example of a relationship that lacks meaning. Sure, the fools around a lot with Rito but is there any sincerity in her desire to be with him?
At least in the parts of the manga I have read (which is until Yami transfers back from her lewd self in the Darkness series) there is no romantic development between her and Rito. Does she learn new thing about him? Very rarely. Does he learn new things about her? Never.
And heck, she even sees herself as so unlikely to be together with him that she starts the whole harem plan just to be part of this harem and get at least a little affection from Rito - which is not even pointed at her as an individual but as her as part of the harem he has to attend to.
Even her delusions are all of naugthy nature and protray Rito as a beast of lust instead of as a partner.
She has found her emotions for Rito and then decided to tranquilize them by starting the harem plan.
Sure, Akeno says she is third - but she wishes to be Issei's number 1.
She wants him to give her personal affection. And while she on the surface accept Issei wanting to be the harem king she is still not too fond of him fooling around with other girls.
There is a foundation for a serious relationship here.
In the end it probably simply boils down to personal preference. I don't like one dimensional characters and I feel like it makes a character way stronger to see how much effort they put into upholding their strength and how vulnerable they can be even though they normally seem so strong. I like human characters.
A good example would be Kamina from Gurren Lagann: I would have hated this guy for being an empty shouthead if he had not admitted to be faking it the entire time.
He is not simply "born a badass" - he is worried and sometimes even afraid but forces himself to be loud and manly in order to boost everyone's spirit.
His core is not made of steel unable to be shaken by anything - his core is soft like that of any human. But his will and determination are strong enough to give him a thick, hard outer layer.
And I think he is stronger for it.