Because she ramps up the physical touch when her internal monologue veers back to Hikari - she's thinking "I will not let this place be taken away from me" right before she initiates the heavy make out session.
Now I understand where you get this idea. However, if you read the WN chapter, this point definitely doesn't hold:
1. Yuu suggests that they could go somewhere else most likely because he is actually worried that she might be bored with him or anything. She responds internally with thinking that he doesn't even have a clue how irreplaceable these moments actually are for her, how it's not unpleasant at all, how fun it is for her to stay like this with him.
2. She initiates intimacy basically to assure him that she likes to spend time this way alone together.
3. The line about "protecting this place" comes long after they started making out. She doesn't think about it before initiating the contact, just as one of afterthoughts.
That's sorta my point though - if he's actively in a relationship with Aya, why avoid questions?
Because revealing such thing to your long crush who also happens to be your childhood friend will cause nothing but uneasiness and embarassment. I wouldn't want to talk about my new relationship to someone I've been in love with for a long time.