Let's hope there are plenty of other more harmonious aspects between you, because with this one, you find yourselves separated by age, and on opposite sides of the fence when it comes to the subject of duty and responsibility. This is frustrating enough in realms that exist outside your relationship -- different work ethics, for example, or senses of familial duty or convictions about money gradually erode your sense of trust. Within the realm of your romance, any disagreement in this area is a real and immediate problem.
Differing needs for emotional security lead to a standoff, fast. Anger and depression are sure to follow. If one of you wants monogamy or another commitment but the other doesn't, you probably each try to force your own way, and you both feel pressured. If this continues to be a problem over time, you can expect only a compounding of the problem, partly because you aren't able to agree on how to fix it. One of you might want to get professional help, while the other wants to work it out at home, together, in private. Finding common ground in these areas feels impossible, because the stakes seem so high. The relationship doesn't necessarily fail with this influence, but lots of compromise is necessary. You might end up having to just go back and forth rather than finding common ground, because balance is difficult to achieve.