Obviously, there are moral boundaries to everything (unless you believe otherwise), but our current society DOES create moral boundaries for every aspect of our lives, even sexuality.
What would you yourself deem "inappropriate" sexual behavior in public? What would you deem "immoral" sexual behavior, no matter what?
I believe sex is only moral when done between a man and woman who are married to each other. When married, it needs to be done with mutual consent, respect, and love.
This means that pre-marital sex, extra-marital sex, rape, incest, sexual abuse of a spouse, pornography, masturbation, polygamy. homosexuality, sex with animals, and lust are all sexually immoral. Men and women should dress modestly in a way to not promote sexual immorality or sexual thought.
By sex, I mean any physical contact that arouses one sexually. Holding hands, a simple hug, or a simple kiss are not sex. If physical contact crosses the line from simple physical affection to passion, then it's probably crossed over into sexual immorality.
I don't know that there is an age gap at which marriage becomes immoral. For example, it's not sexually immoral for a 25 year old woman to marry a 55 year old man. Now if the 55 year old leaves his wife of 30 years, without good cause, to marry a beautiful 25 year old, that's another issue and is immoral.
I'm not sure what to think about what age is too young to marry, from a moral viewpoint. My mom was 16 when she married my dad who was 19. That was young and most would say today that it's not a good idea, but it's not sexually immoral. (They have now been married over 50 years).
Sexual immorality comes in lots of hues and shades. Some sins are greater than others. For example, rape or incest are obviously worse than sex between consenting non-married adults.
I believe that sexual immorality is one of the greatest problems facing the world today. I know there are wars, drugs, gangs, and violence on the streets. I know there are a lot of problems in society which are very serious. I believe that sexual immorality ranks up there among the most serious problems.
Sexual immorality often leads to disease, unwanted pregnancy, abortion, low self esteem, broken families, broken hearts, and fatherless children. This can lead to dysfunctional homes where children are not properly taught and have no good role model. This can lead to drug use, gang participation, violence, disrespect for women, and more sexual immorality. And the cycle goes on and on and on.