Well, as far as the sexual attraction to other women, I think Dan and Jennifer put it best... people are just sexual. We're too big on labels like straight, bi, gay and lesbian... people are sexual, period. It's no big deal to be attracted to someone of the same sex... although as a society we for some reason feel female on female is good and male on male is not... I do not subscribe to this... sex is sex, no matter who with (wish safety being a precaution always, of course
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As far as the relationship issues. I can most certainly understand your hesitation on trusting him fully again. That trust was broken, and once broken, it can be very difficult to regain. Having him trust you is good though, and it sounds as if he does. You obviously trust him enough to have remarried. As for the just watching and not joining... well... lol... I think most every man wants to see 2 women together, and usually wants to be with both women also... it's that stereotypical porno fantasy. Would you be offended if he decided he did want to join?
Whenever introducing a new partner to the bedroom you need to be sure to talk. Talk with your hubby before hand. Find out what he feels is too far.. explain limits for both of you.. what can and cannot be done, and where this may or may not lead to. Talk to your new playmate and tell her these rules. Then while playing, if anything is weird or uncomfortable speak up... say so. Then after you'll want to talk it out as well. This all goes for a 3some, 4some, or if it's just you and another woman... feelings and emotions can get in the way and mess things up if they're not communicated.
I think this can be a very fun and potentially relationship building experience for you. If all goes well, you may well find your intimacy level with your husband going beyond what you have ever had.
Good luck and happy sexual explorations!