You do have a number of positive and harmonious energies between you, although as with all relationships, there are lessons for both parties to be learned in this union; the purpose for relationships is to allow each to grow and progress as individuals.
There is a dynamic opposition between his Sun/Mars and your Venus/Neptune, representing a significant source of contention or disagreement; his sense of Identity can at times be undermined by deception or mistrust on your part; or perhaps your own insecurities make you more demanding of him. His Sun in Gemini expresses quite differently from your Sun in Scorpio, suggesting the potential for power struggles or issues relating to control. Here you need to find balance between the Sun in Air, while yours is in Water and that combination can be difficult to find unity in, without mutual understanding and effort. In both cases, the Sun highlights the significance of the 4th H, relating to emotions and experiences of the past in terms of how your early upbringing influences your relationship now.
With the Venus and Mars energies at odds, this too contributes to disagreement or conflict.
There may be times when you feel he is not living up to your expectations; with Venus and Neptune involved this might suggest potential betrayals of trust, lack of faith or suspicions that can interfere with maintaining unity; trust, sincerity and faith are all connected with the aims of Sagittarius; Sun in Scorpio is often challenged by emotional intensity, jealousy or obsessive tendencies and lessons are largely the result of personal insecurities or fear.
Sun in Gemini is typically much less emotional and more superficial, by nature, than the Sun in Scorpio: do you ever feel that he is perhaps downplaying the significance of your emotions
or minimizing your feelings? Does he have difficulty with your probing, penetrating manner of dealing with emotions.
I am sure that you are aware that undertaking a full synopsis of a your relationship requires a great deal of time, effort and $, so these are just some brief notes of what could be problematic in your relationship; You are coming towards a specific cycle of Saturn, known as the 7-year itch, which is perhaps the reason for your queries at this time. Thus, at this time the question is about what you feel may be wrong about the relationship, and what is it that keeps you together.
As per marriage, that analysis is also very complex, and requires paying attention to the upcoming cycles and transits that will trigger your personal planetary configurations into action. You might be more specific about what it is you are wanting to know about.