Well, first off, I am glad to hear that you have taken such good care of yourself! Fitness and diet really are the fountain of youth! You do, in fact, sound like a considerate and caring partner. It is great to hear that you have found a partner that you adore as you describe. Being sexually fulfilled is important and reciprocation and consideration are definitely key parts of a fulfilling sexual relationship. I will address the sexual aspect of the relationship first, and then we can move into the thoughts patterns you are feeling, and then the aspect of your feelings regarding her physical health.
The sexual response cycle has four phases: excitement, plateau, orgasm, and resolution. Both men and women experience these phases, although the timing usually is different. I will address more of the resolution cycle, because that seems to be where the issue lies (with her)....During resolution, the body slowly returns to its normal level of functioning and all of the muscle tension that led to the orgasms is released and those fantastic endorphins, that make our sexual experiences so bliss, lead us to a general sense of well-being, enhanced intimacy and often fatigue. So, she probably is being very truthful in feeling relaxed after that great deal of pleasure! Is it possible, to address with her what your needs are? You describe a committed relationship, so the communication is key. I know you have "broached the subject" but I wonder if you take the time to explain it with vulnerability, rather than what she could possible perceive as criticism, it would be helpful. Maybe try discussing in how it is an emotional concern for you? Something like "sex is a very important act between two committed people and that is why I enjoy making love to you as much as I do. I want to provide you pleasure, and while that is enough for me, I would like to ask how we can both ensure that our needs are met completely." Also do consider that you may really not be that concerned about the weight gain, but you may just be frustrated with the lack of sexual reciprocation, that you may develop a slight negative filter in how you perceive and process her. I would also encourage you to think of how you guys can experience more quality time together by engaging in physical activity if you feel that is a concern..... walking, playing golf, nature trails, hiking, biking, etc.... Even engage in cooking classes that focus on healthy eating together and take the time to prepare meals together!
It sounds like she offers a great deal of affirmation to you through her words. Something important for all couples to consider is understanding each others' love languages. "The Five Love Languages" by Gary Chapman is a great book that helps us understand our own love language and our partners. I am going to include a link at the bottom of the answer that you can look at that will be helpful in this area. Basically the book suggests that there are five ways to express and experience love that Chapman calls "love languages": gifts, quality time, words of affirmation, acts of service (devotion), and physical touch (intimacy).
This is another link that is somewhat candid, but key points are addressed:
I hope that you will take the time to accept this answer and provide a positive rating. I am available for further processing as needed on the topic, even if you accept the answer! Best wishes! Julie