Agree. If I have a formal relationship with you, or I'm just a casual not-so-great friend, I understand receiving messages at any convenient time. But with loved ones, especially if they've seen the message, a no-reply within a reasonable time would just indicate lack of interest in me. Of course they might be busy, in which case I'd expect a response indicating the same. Mobile communication is easy, it just takes some etiquettes you previously hadn't learned.