Filled with testosterone the younger man is filled with hopes and dreams along with a great deal of confusion.  Sexuality is repressed in many societies and sex education lacking in many countries,  yet the use of sex in advertising as well as the porn that is readily available can distort his perception and expectations for sex and relationships. Then there are some born to be good lovers while others remain inept.

In many Western countries, sex education is improving, parents are becoming more factual in talking about sex, emotions and relationships.

If it is both yours and his first time,  share the responsibility,  take it slow and talk about what you are feeling both within yourself and about each other.  When it comes to the actual intercourse part, sometimes getting the penis into the vagina can be difficult.   If your pussy is to tight, or his angle of entry is wrong, it can really hurt or just not go in and lead to some  embarrassment.

If you are sexually experienced, the freshness and innocence of a virgin male can be delightful and it can be a pleasure to guide him to pleasure you.

There are no real stats,  though from my experience there is nothing like a fresh younger man - 16 - 20. They tend to be more playful, have lots of stamina, and although emotionally immature, fun to be with, eager to please and take direction well.  Older men when set in their ways, can be a bit stiff and less playful, but when well experienced,  better lovers.

If you are inexperienced, by all means entertain young men, but be prepared to take charge if they are not meeting your needs or cannot cope in the situation.

It takes time to learn to where and how to touch.
It takes time to learn self control so as not to cum to quick
It takes time to learn how to give and receive.

Questions
I am a thirty- two year old female and am engaged to a loving nineteen year old male. He has some past issues with his penis size, and feels that he is inadequate. His penis is just short of 7 inches and fills me up with no complaint at all.
This is a problem common many men face and they may have other anxieties.

I enjoy sex but can only get off when I stroke my clit.
Achieving orgasm is a problem for a large percentage of all women, have you tried more oral sex?

My fiance now strongly resents that I can give him pleasure when we have intercourse and pleasure when I manually stimulate him but feels that he is too small to make me cum with just his penis.
Has he well explored his options to assist more? How skilled are you as a team?

This is turning into a real problem. He believes that if his penis was bigger he would make me come. I have told him that I have never been able to do that with just penis alone.  He knows that I have had more sexual experience than him and that is obvious because of our age differences. And again have always stressed that it has never happened with anyone and that I am not just trying to be difficult.
My question is this: Can a woman cum from penis only stimulation?
Yes, but less than 1/2 of those who complete our surveys.

Is there something wrong with me? I have heard of the "G" spot and all of that but every woman I have ever known says that they like their clit rubbed too with penis.
yes, feels great,  especially when he rubs the clit with his penis tip

I am willing to do whatever it takes to make this all better..
ok so really you both have to discuss this and him leave aside any fears/issues. 

the bottom line is this: he is starting to resent me and I am scared. he feels that I can make him cum with my pussy but he cannot make me cum with his penis (alone). he is even considering something for penis enlargement, he doesn't need it.... help.... now he just wants things as he says "even"  whereas we just manually stimulate ,this is okay but the best is when i can feel him inside   can u help me???
Have the discussion, or get joint counselling, and you must find a resolution.  Buy him a months supply of penis enlargement pills in support.   With a good teacher, Tantra is an excellent way of exploration which would help relating and introduce new ways of having sex that may surpass some previously very good orgasms.