Frankly, yes.

Not because it's guaranteed that anything untoward is happening, moreso because she now knows that this is fundamentally a trust issue and rather than address it she seems to have shrugged it off.

If the tables were turned here, would you not go the extra mile to ensure that she knew the long and short of the texts?

Sure, she denies it. Fair enough. But she has done nothing to ease your doubts. That's what makes my internal alarm bell ring.

Last night, I caught her lying to me.  I saw her texting one of these guys, so I asked her about it.  She denied that she was texting them.  So, I asked to see her phone and sure enough, she had texted one of them.

I still don't think she's cheating on me, but I don't understand why she's hiding this from me.

Why are you denying the obvious? Do you think if she was into you she would be acting this way? She is playing the field so don't be surprised if one day she says goodbye when someone comes along that is  "the one".

I was in a similar situation, and the best advice I can give you is trust your gut. We tend to be very intuitive when we listen to the signs.

If you have confronted her about these conversations, and she has told you they are just friends and nothing to worry about, you have two options.

1. You trust her. Has she lied to you before? Has she been flirtatious with other guys before? If you do not have any explicit signs that she is lying to you, you are going to push her away with your distrust. Your second option is:
2. End the relationship. Once you stop trusting someone, it is difficult to get that back. If she is showing red flags, it is up to you decide whether or not they are deal breakers.

Simply put, if you cannot trust your significant other, it is best to end it before it progresses and causes you both unnecessary pain. I know it is hard to put your trust in someone else, but it is imperative to make your relationship work.

Best of luck.

This is an annoying situation but it is something you need to take control of. Simply talk with her and tell her how you feel about her texting other guys. If it really bothers you, tell her that. Tell her you just feel uncomfortable because she never mentions the guys to you.

If possible, make it known to that man on the other side of the line that you exist.  Perhaps during house parties, or just a double date.  Get your girlfriend to arrange for it, if there's nothing between them, what is the problem. 

Warning, if you cannot resolve the doubt, do not go deeper into this relationship.

Wish you well.

hmmm ok good luck bro, I hope you find the one.

If she continue to behave in a manner suggestive of concealing something, you may have valid grounds to be worried.

I'm in the same / similar situation,

...new girlfriend of 4 months, had an affair before we got together. Have since been in social gatherings with the same guy (whom is married and has two kids, but his wife has no idea), first social gathering with this guy - he was very weird towards me, just starred and no conversation, so had to break the ice and chat, other people noticed this behavior also.  He sent me a ridiculous apologetic plee email, saying "what close friends we are" blah blah, but he did not mention he had been having an affair with my new girlfriend.

New girlfriend has not mentioned anything about the affair with this guy, she has just told me this guy (of two kids and wife) has been "creeping on her".  Think she is in denial over her history with this guy, but clearly some of her friends are aware of the full story, just no one (including her!) has been honest about this situation.  They are still in contact and I have seen some of the messages about how he misses her and the "chats" they used to have "not the other stuff", but she appears to not reply with the same intention, but the whatsapp channel for historic communication (sex chat) is still open. For all I know they chatting all the time, she is just very good and deleting all the recent messages between them, I just have to suck it up and not let my mind play games on me.

Have confronted her about it but she gets very defensive and turns into a small argument, so I no longer mention it, I don't mention anything about it anymore.

Obviously frustration lies with the fact she still has contact with him, but I have to let it go / let it happen otherwise it just ends up in argument, and it pushes her away, so just playing it cool for now.
 Weird though knowing (that's how it feels at least) you are not the only one competing for her affections, so it does feel bit of a love triangle. 

Maybe I should mention about hanging out with other females and see how she reacts?