Triangle Blue

NTR or Netorare. Typically, steal the girl protagonist of the story. 

These two provides same Netorare story which contain: 1. Dumb stupid girlfriend who easily give her self up. 2. Beta careless boyfriend which can't do nothing. 3. Charming third-person whose aggresive. 

main two charcters get raped 

Many girls get "stolen" from army guys 

Both titles are classed as NTR or Netorare(Cuckold) which is a sub-genre in hentai and delves into the betrayal from the main leads partner. So for those unprepared, be warned that each title will usually get a rise out of you(and not the good type from down there) as in most cases, its usually in the form of the girl having sex with someone else whilst her partner watches defenseless all due to his own insecurities. If this is your fetish then I can guarantee that both titles will aim to please. For more info on these type of hentai, visit The Netorare Club here in MAL. read more 

Both of these titles feature a pre-established relationship getting torn apart when a random guy steals the main gal (netorare). Additionally, both are based on games by LiLiM Darkness and sport similarly designed heroines. 

Characters are very similar and setting is almost the same, except that Triangle Blue has a "good" ending.