When both the male and the female are Latter Day Saints, dating morals and expectations are very similar and don't have to be explained or justified.  That's just the way LDS dating is.

For a young man who is not a member of the LDS, dating a devout LDS girl means she must either tell him constantly when his speech or actions are outside of what is acceptable to her upbringing or she must fight quietly with her conscience and beliefs every time he does something which is well within the acceptable behavior by his standards, but is considered to be disrespectful at best by her standards.

If you take the element of religion out of the equation, the young man enjoying LDS dating may not understand why she would not agree to a session of necking.  At the same time the young woman is frustrated because she may really like the young man but can't understand why he doesn't keep his hands off her.  As a result, she feel disrespected, but yet intrigued by the action and having been trained to be less forceful than her compatriots, both parties may find non-believers and LDS dating to be a very uncomfortable experience.

Even if this couple makes it to future dates, the dilemma doesn't go away.  The young man of 19 to 21 who is not used to a LDS dating expectation may very well have no intentions or expectations beyond the moment.  He's looking for a fun date to do something now and maybe tomorrow or the next week.  For the young woman, on the other hand, LDS dating is intended to get to the point of marriage and motherhood.  So, for the non believer, dating can often be an end in itself, while for the LDS dating is a means to an entirely different end. 

In all honestly, most LDS church leaders recognize that even 'true love' is rarely strong enough to survive two such radically different approaches to dating, let along marriage.  Because people tend to marry those whom they date, the church leaders have taken the stance that LDS should not have relationships which might be expected to lead to marriage, outside the church.  Even if the young couple manages to meld their beliefs in such a way as to prevent disagreement and conflict, it is unlikely that the parents of the LDS child will be accepting. 

Assuming both parties hold strong beliefs in their own religious doctrines, it would seem that neither a young man nor a young woman should be dating with non-believers. For the non-believer to understand that to a devout member of the LDS, he is a second class citizen, never able to be fully accepted in her church.  She may be ostracized at some level in the same way that he is. This varies in different locations, but is still very difficult to bear.