Since you've directed your inquiry toward "religious Christians," and I consider myself such, I'll try to answer your question as it relates to me.
I don't really connect the commercialism that surrounds the Christmas holiday season with the celebration of the birth of Christ. I know that in a lot of ways, they're all connected, but it's just not the way I think about it. The holiday season provides a much needed reason to reconnect with family you only see once a year, and to take a break from hectic work and school schedules to spend time together.
Celebrating Christ's birth at this time is almost coincidental to my mind. At our church, we always prepare and sing a Christmas music program with choral pieces that communicate the details of His birth and what His life eventually came to mean for mankind. We also schedule a church family Christmas dinner early in December to eat and share gifts with one another. The kids often present a little program, we tell funny stories and sing funny songs, then we sing carols, and our pastor presents a Christmas-themed challenge. As brothers and sisters in Christ, we have a familial bond that is as strong as - and for some, stronger than - our relationships with our blood relatives. When Christmas Day falls on a worship day (Sunday or Wednesday for us), we always have a service. A couple years ago when the 25th was a Sunday, it was such a joy to spend Christmas day with my WHOLE family, not just my kin. We also usually celebrate communion (the Lord's Supper) close to Christmas time as a reminder that He wasn't just born, but also died to pay the penalty of our sin. This year, we had a Christmas Eve communion service that was a great blessing.
It is true that, while we often allow ourselves to get swept up in the pomp and circumstance of Christmastime, the event we celebrate that has the most spiritual significance to us as Christians is the resurrection. Christ's death and resurrection were His entire purpose in being born as a human, and the accomplishment of our salvation.
As far as whether find it insulting for folks to celebrate a secular Christmas, no, I really don't. The Christmas holiday is a huge part of our modern culture. I am not easily offended in the least. The only "secularization" of the holiday that bothers me is those who are militantly anti-Christianity, who want to prevent people from wishing others a Merry Christmas, or from sharing what Christ's birth means to them, or from displaying a nativity scene.
I was happy to notice how many people pointedly wished me a Merry Christmas this year. A few years ago in my area, local merchants were warned not to wish people Merry Christmas for fear of offending non-Christians. I have no objection to being greeted with Happy Holidays or any of the other generic holiday greetings. It's just nice to hear it again.
. I have it on fairly good authority that eating in the same room as someone fasting (by choice!) isn't disrespectful. So on the one hand, those with a strong anti-Christian sentiment think you should abandon Christmas, or its religious meaning at least, and in the same breath you're advised to observe another person's customs and traditions (which is odd, to say the least! 
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