If I send out a message to someone, I don't assume that they owe me a reply back or that if they don't reply they're being rude.
There are many reasons why someone won't reply;
1. They may not be a paid member. I think in the past people who hadn't paid to be a member of the site couldn't send an emote--even a blank emote. Now they can. When I wasn't a paid member at different points, I assumed I couldn't send an emote back.
2. They don't want to mislead someone into thinking they have an interest in them. I did sometimes send out a smile emote after I realized that I could do so without being a paid member. Sometimes people would respond right away with a message that I then could not read. After a few responses like that, I thought it was best to not even send a blank emote, if I wasn't able to continue the conversation. Not everyone is up on the changes in the site. They might not know that non-paying members can send a smile or that they can't read messages sent to them. At one point, I think people were able to read messages, but not respond.
Also, as Victor mentioned in an earlier post, people can take even clear replies of non-interest to be an encouragement to "try harder". There are only so many times you can say "not interested" before realizing that you have to stop replying at all.
3. Some people are married and still have a profile here. For whatever reason there can be outdated profiles on the site. I know that a couple people occasionally came up on searches, or on the home page after they were married and no longer active on the site. Who knows why their profile wasn't removed? Maybe they chose to not have notifications sent to them about messages and forgot they hadn't removed their profile. Maybe the email that they linked their profile to is one they no longer use and they've forgotten to remove their profile.
4. Sometimes there are things happening in real life, and we can't respond. When my Mom died, I was a paying member here. She died suddenly, and my Dad needed lots of care. The last thing I was worried about was replying to messages on CM. Even when things got back to a routine, I didn't have the emotional energy to respond (at times).
I've been a paying member on and off for 6 or 7 years. When I've been a paying member, there have been many instances when real life "stuff" kept me from replying to messages... I was in the process of moving both my home and studio, and didn't have the time or energy to spend on CM. After moving it took FOREVER to get internet set up at my new place. Trips to the library to use the internet were used mainly for business correspondence. When I'm prepping for a big art show, and running on little sleep, I don't have the energy to be corresponding online.
5. They may have a lot of messages to reply to and find it hard to catch up with replies. Yes, it can happen. Off and on, I've had a profile on a free site. Women can get swamped with messages when they have a profile there--especially when they're new to the site. I've had pages of messages to answer, and every time I went on the site more people would message me (it shows up somewhere that you're "new" and "online"). As I would reply to 5 or 6 messages, 10 more would come in (often from people I had just sent replies to). I had pages and pages of unanswered email within a few weeks and it took a lot of time to answer them. I put "Any good Catholic guys out there?" in my tag line, and got MANY angry messages from anti-Catholics. Lots of people sending me rude emails about the abuse that has taken place in the Catholic church. I especially tried to respond to those emails with polite messages back, but it took a LOT of time, and I didn't reply to every message.
6. Sometimes the messages don't warrant a response. Do I really need to reply to "Hey, you're cute/sexy. Wanna hook up?" ? This sort of message was more prevalent on the free site, but there have been versions of it on CM as well.
To sum up, we really don't know for sure if people have even received an email. If they have, they don't "owe" us anything. If they don't reply, it's not necessarily rude. You have to have a bit of a thick skin to be on online dating sites. The best experience is when you don't take yourself too seriously or make the assumption that every person's inattentiveness is a reflection of your worthiness or theirs.