I think this manga, and others, reflect a change in consciousness that is ongoing in Japan. What used to be played as subtext, or covered in symbolism (Utena), now is being faced. I think if Nanoha was remade today they wouldn't shy away from marrying Nanoha and Fate. Yagakimi got an anime and has done really well. This manga exists in the context of modern day Japan, not Europe or the US.
If it seems preachy, I think it's because we come from a place where the issues are more worked out. Not perfectly, but to a greater degree for sure. Keep in mind that same sex marriage is still not available in Japan.
Interestingly an anime that was basically the modern Nanoha just ended, you may or may not know it idk. Senki Zesshou Symphogear is an anime series with a number of similarities, and flat out inspirations, from Nanoha. It's got high tech magical girls who are genuinly badass, it also has LOADS of gay subtext. In the final episode of the show much of that gay subtext comes to a head, and all plausible deniability basically goes out the window, it's to the point that anyone denying it is absolutely full of shit. That said, there is no kiss at the end, and the show cuts off right as a confession is about to be said,but again it isn't ambiguous at all, the audience knows it is a love confession and the creators know we know, but they still didn't show it for some reason i can't quite grasp.
Anyway, I think that's pretty indicative of the weird ambiguous place a lot of lgbt representation is in Japan right now.