No it is not a sin to not get married nor is it a crime but I guess it could be a shame but if you're happy with your circumstances , then more power to you. Many people never get married for a huge variety of reasons and are just as happy and content as anyone can be in this world. Many couples live together married and love it that way. Marriages while still around, are fast unfortunately becoming old fashioned. I've been happily married over 35 years now and its been a blessing for me and would do it all again if I had to redo my life over. So do what makes you happy in life as you really are the only person you have to impress and don't worry what others say as its your life, not theirs.
Do your own thing and be happy is all any of us can hope to get out of this life. Good luck