I think you have good instincts and know what is going on here.
Before my husband and I married he was in the UK for six months before coming over. Then, after he went back he was gone for another 4 months before he came to stay.
I spoke with him every night...even when I was tired I made an effort because I missed him so and he felt the same. We did do things with other people but I would NEVER have gone out with another guy...even if it was just a friend, because I know how he would feel about it. I also don't think it would have looked very appropriate to other people...especially those who knew the both of us.
Have you told her this bothers you? I would and see what her reation is. If she poo poos it then you have your answer. Any woman worth her salt would be a bit considerate toward her mates feelings.
Thats nice that she always says you will always be hers and that she loves you so much...her actions should match her words.