I've hesitated to comment, because I wanted to make sure I could communicate my thoughts clearly. That said,
My husband is similar in that he trusts his gut a bit more than I am often comfortable with. Some of that is because we are different in that regard, he tends to take people at face value and go with that, and I tend to trust no one fully until they've earned it and that can take years. Which doesn't mean that he will go willy nilly into any situation and it doesn't mean I don't let people in at all. But I do tend to take almost everything people say "with a grain of salt", until they've stuck around long enough for me to see if their words and actions are in tandem. Even then, when it concerns health issues and life changing/altering things... it takes a whole hell of a lot for me to trust someone in matters that have such ground changing affects.
L reminds me of a few friends I have had in my life. Love them, but am no longer close friends because they breed drama into their lives. They make choices that bring drama, choose people that bring drama, choose careers and livlihoods that bring drama. If there's no drama, they get restless. When you're too close to that, drama ensues and sometimes you can get caught up in the whirlwind. It took me a while to see that what they said wasn't always the truth. Even longer to see that many times they didn't know they were being dishonest, their view of reality was so skewed that facts were different in their little world. This is probably part of the reason I don't trust very readily, actually.
I think maybe, once you get past the immediate drama, there are a couple of good talking points (I hate to say lessons, because that sounds like you're children)... for you and G to discuss seriously. Like if you're going to go without condoms with someone, when would that be, how would you discuss it, what would you require first... that sort of thing. Having these agreements in advance of NRE is important (and knowing you have to stick with it until you talk might help get you through the nekkid horny bad decision time period!) My hubs and I call these our "times of growth"
. They can be good things coming from craziness, they can advance your communication by light years and they can make you stronger as a couple.
As far as L and the pregnancy... definitely keep an open mind. But I would definitely not do anything without proof. The fact that you asking her questions makes her "defensive" shows either that she's hiding something, or she is just very immature. I believe any sensible, mature, grown woman would understand that a situation such as this would involve a lot of discussion and questions and clarification.