I think Adam nailed it. Men will, by nature, notice an attractive female. As much as we hate it at times, it just sort of...happens. But physical attraction and love are two totally, completely separate things. What you're asking for, if I'm understanding you correctly, isn't only NOT unrealistic, it should be pretty much the standard for a committed relationship. His eyes might catch an attractive woman from time to time, but he should always hold you in his heart.