Okay so here's what happened just to be clear:
- Hinata got curious about creampie sex (or maybe just raw, I'm not sure because of Google Translate) after hearing from Miku and Kanaria how good it feels.
- Hinata tells Sou they'll do it raw once, but Sou has to cum outside. Hinata is also still taking birth control pills.
- They do it once once raw with Sou cumming outside, but it felt so good they did it more.
- Well, that still felt so good that Hinata didn't mind Sou coming inside once, and because Hinata thought she's taking the pill so she's safe.
- That "once" became many times because it felt good and Hinata felt she was missing out all this time.
- And then some narration afterwards after Sou came again inside, probably from Hinata, about how "they didn't know yet at the time" and something about Hinata or her family about being prone to being pregnant.
So yeah, that's where I inferred that
Hinata is the first one to get impregnated. I take it back when I said it's confirmed, but it is strongly implied. The recent pages haven't shown Hinata again yet since he and Kanaria is/has trained Maria to actually last more than a second.